Center Controller reported an aircraft lost radios and transponder became inoperative. The aircraft deviated from its route and into conflict with another aircraft.

Narrative

While working the R-side position at 66/67 I received a call from ZZZ1 Departure at roughly XA:10 Local. They coordinated that Aircraft X 10 NM S of ZZZ Airport was a primary target only Aircraft X; NORDO; possible equipment failure; on course ZZZ Airport. ZZZ1 Airport last assigned altitude 5;000 ft. No reported altitude. ZZZ1 Departure said they never talked to the pilot. He was NORDO from ZZZ2 Center so they weren't sure if he was an emergency or not. I called Aircraft X radar contact. I immediately passed this info on to the Controller In Charge (CIC). Around 5 minutes later; Aircraft X turned off course to the west; maybe a 10-20 degree right turn. Info passed to CIC. Around this time the traffic in the sector picked up and became complex. Multiple aircraft needed extra spacing and needed to swap out as necessary; which; combined with military airspace R-XXXX SFC-FL400; Location MOA 070-FL400; and Location 1 MOA 070-150 being active and covering half the sector; required that most of my attention be put towards traffic other than Aircraft X.There was no D-side available due to staffing; though one did arrive 15-20 minutes after needed; so I made all required coordination with Aircraft X in between necessary clearances to other aircraft. By the time I had a chance to call ZZZ3 Approach; Aircraft X was roughly 20 NM NW of ZZZ2 Airport. ZZZ3 Approach advised they could not see the primary target; nor the tag; and therefore could not take a point out or handoff. Aircraft X continued to track SW-bound; off course; as a primary target only. About 4 minutes later; when Aircraft X was around 8NM NW of ZZZ2 Airport in ZZZ3 airspace; Aircraft X and Aircraft Y; Aircraft Y talking to ZZZ3 traveling SE appearing to be on an IFR clearance with ZZZ3 indicating 5;400 ft. got within 5 miles of each other. I do not know if any action was taken by pilot or controller. 5 miles is my separation requirement; ZZZ3 may have different requirements. Eventually; Aircraft X went CST Track (Coast Track); [radar] no longer tracking on my scope; I reported the location to the CIC; and called ZZZ3 to advise them. They let me know that a tower; possibly ZZZ1 Airport Tower; was talking to Aircraft X. Better staffing; with the intention of a full-time D-side in the area; may have allowed for coordination to take place place earlier with ZZZ3 and ZZZ1 Departure; giving us all a chance to come up with a better plan.