Air carrier Captain reported an EFB malfunction; FMC malfunction; and associated Autopilot malfunction during descent. The pilots brought the flight director back online and continued to landing.

Narrative

We were operating from ZZZ1-ZZZ2 on Date and had a possible altitude deviation during our descent due to an automation issue with the aircraft while on descent on the STAR. The flight was the second segment of the pairing and prior to this event we had no issues with the flight to ZZZ1 with the same aircraft. While in ZZZ1; I noticed the high forecasted winds out of the west in ZZZ2 and decided to add 10 minutes of extra fuel for any unforseen delays. After receiving our clearance we were notified of a 40 minute EDCT (estimated departure clearance time) due to the winds in ZZZ2. I made a quick phone call to Dispatch to confer and verify that the winds were the reason for the EDCT. We worked on a plan to push 10 minutes later from the gate; which would allow time for a Southeast Taxi and possible 5-10 minute delay at the end of the departure runway prior to being cleared for takeoff if everything went right. Everything did go as planned and shortly after takeoff we received an ACARS message from Dispatch alerting us to possible 20-30 minute hold times inbound to ZZZ2 due to winds. Upon reaching cruise altitude we formed a contingency plan between ourselves and also advised Dispatch of our limited hold ability. We were nearing our descent phase of the flight and as per my preflight FA (flight attendants) and PAX brief; I had briefed our FA on the ground that we would be seating them early and to have everything stowed with the FA in their jumpsuits at least 30 minutes prior to arrival. We took one final break about 50 minutes out and I reconfirmed with the FA that they were to finish up within 20 minutes and be in their jumpseats in anticipation of the reported turbulence in our descent. The rest of the flight went well till the shortly after we started our final descent.During our brief we had planned for the contingencies of what we would do if we had to land on a north runway with gusty crosswinds. My First Officer was a probationary pilot and was the flying pilot. Given he had prior experience in gusty crosswind landings in the aircraft; but was still relatively new; we agreed that if crosswinds were 30kts or greater on a North runway I would land the aircraft as that was his personal comfort level; however if they were less than that or if it was a West landing runway my FO (first officer) would land the aircraft. After briefing the approach for Runway XX; covering contingencies for multiple runways and finishing our descent checklist we started our descent into ZZZ2 on a Descend VIA the ZZZZZ RNAV landing North about 5 minutes later. We rechecked the landing data and current winds and saw that the winds were around 23 kts crosswind for Runway XXR and I asked my FO how comfortable he felt with that to which he said he was comfortable landing in those conditions. I told my FO to continue as the pilot flying in that case and I would do the runway change in the FMC. We were somewhere in the mid FL200 and started into the light to occasional moderate turbulence at this point while we were somewhere just prior to ZZZZZ; perhaps less than 5 miles ZZZZZ; when we had the automation issue with the aircraft right before I could do a runway change. We were in FMC SPD/VNAV PATH per our FMA with a bottom altitude of 11000 ft. in the altitude selector and around 280kts when suddenly the our autopilot disconnect alert came on and we found ourselves in CWS (Control Wheel Steering) and CWP (Control Wheel Panel) modes and both of our flight directors missing as we approached ZZZZZ intersection. After the initial startle factor I told my FO to hand fly the arrival while I tried to trouble shoot the issue. I believe my FO initially tried re-engaging the CMD B autopilot first when this happened; followed by my trying to re-engaging VNAV/LNAV with no luck. I then tried a quick reset of the Flight Directors (and noted the Captain side would not take the master leading me to wonder if it was a CA (captain) side FCC issue) which did not work either. During this time my FO told me he didn't know if he could make the altitude restriction at ZZZZZ as we were somewhat high so I called ATC and asked for altitude relief due to a flight computer issue. We were given the relief as well as offered any assistance such as a heading if needed. Other than being higher than planned profile due to the startle factor we seemed to have everything under control so I advised ATC we would let him know if we needed further assistance but okay for now. At this point I told my FO I was going to check the QRH (Quick Reference Handbook) real quick too see if I could find a quick solution only to quickly remember my iPad was at less than 15% (while in dark mode and remaining off most of the flight) due to the fact that the aircraft had no USB charging port and that the only power charging station was behind my FO head. I had been charging my iPad during our 50 minute turn and during our flight but it never went above 30%. My FO also stated his iPad started at 100% as well and while plugged in the entire flight he was already at 30% battery life. I quickly grabbed the paper QRH; did a quick search and did not see anything immediately that would help us solve the issue without more trouble shooting. Given my long day; fatigue starting to set in and the time of the situation I put the QRH back and told my FO to just continue hand flying the aircraft as it was probably going to take too much time to find the right section of the QRH with the turbulence and workload level of our flight. After putting the paper QRH away I then asked my FO if we would like to try level change at least while manual setting in the altitudes to see if that would do anything; to which he responded yes. Once I hit level change our vertical guidance on the flight director came back and noticing that; I engaged heading and our lateral guidance came back as well. I asked my FO if he was comfortable with me engaging LNAV/VNAV to see if it would re-engage and after his concurrence I did. We then recovered our Flight Detector and were able to re-engage our autopilot. After taking a few moments to make sure we were where we were supposed to be and doing what what we were supposed to do on the arrival; I finally had a moment to ask my FO if he would like me to load up runway change again. After his concurrence I did the runway change; verified the changes with my FO and hit execute. Once again; right around ZZZZZ intersection; our FD disappeared and the autopilot disconnect alerts chimed and we were also once again in CWS/CWP. We quickly went to LVL CHG and HDG selected followed by LNAV/VNAV to bring our flight directors back up followed by re-engaging the CMB B autopilot once we had made sure the aircraft was doing what it was supposed to be doing on the arrival. After this we decide if we had any other issues we would just hand fly the aircraft for the remainder of the flight. The rest of the flight went without incident as were just about finished with the STAR and told to expect vectors for a visual to XXR