nubie here, so take with a bit of skepticism... i was having similar issues with my rexing v1p, the last file recorded before shut down would not play on my computer.
what worked for me was to play that file on the dash cam. the camera has internal software to "fix" the file.
tech support at rexing had suggestions of charging the camera for a few hours in your house, using the reset button and upgrading the firmware but, none of these helped.
in your case, i'd suspect the camera's internal battery is no longer holding a charge. i'm sure someone reasonably handy could replace it, you might search some youtube videos by brand / model.