Volvo S60 Problem Report
Volvo S60 Fuel gauge, rear door locks and trunk switch inoperative
There is a computer that controls all these items and it's called the rear electrical module, or the "REM". To temporarily address this issue you can pull the fuse from the rear fuse box labeled "REM" and everything should start working again.
Volvo has updated software for the REM that will help prevent this from happening again. See a Volvo shop or Dealer and have this and all the software in the vehicle checked and updated every 30,000 miles.