Enroute Controller reported having a communication problem with a UAS. The controller reached out to the aircraft on Guard; reestablished communication and recleared the aircraft to continue operations.

Narrative

Aircraft X was cleared on a 15+00 mission in the restricted area. Aircraft X auto popped on Sector X over the ZZZ VOR. During a busy session. The D-side nor myself had any information about the aircraft or what the were doing. Aircraft X was westbound exiting Sector Y. I asked Sector Y what Aircraft X was doing or if they were talking to them. Sector Y indicated they never talked to Aircraft X. Sector Z to the west of Sector X had Track Control of Aircraft X. My D-side called Sector Z and asked if they were talking to Aircraft X or had any other information and they said no. We went to Guard and called for Aircraft X. Aircraft X came on frequency immediately. I asked them what there last assigned frequency was and was told XXX.Y. Aircraft X was re-cleared in the restricted area. With the level of traffic and volume over ZZZ VOR it is very hard to see limited data blocks and multiple aircraft were in potential conflict with Aircraft X.Restricted areas need to be able to be depicted on the scope to easily identify areas the aircraft may fly into. Aircraft utilizing the areas should only be allowed in one area at a time and request clearance into each area when they are ready to transition from one to the next.Tactical frequency for ZZZ [Center] needs to be re-instated and aircraft such as this need to be on this frequency. Procedures for handing off vs pointing out aircraft need to be based on what area the aircraft is cleared into to avoid the thought that the other sector may or may not be watching.