It is very difficult to diagnose this issue. If it is a firmware bug, a factory reset and a firmware update may fix the issues. But, if it is hardware failure, factory reset and update will not fix the issues.
I suggest you to first try backup all recordings to computer, and then perform factory reset. If factory reset fix the issues, you are in luck. There may be some bugs that caused the rear camera to suddenly stop working. You just need to update the firmware. But, if factory reset does not fix the issues, chance of hardware failure is very high. You may still try firmware update, but the chance it fix the issues is low.
If it is hardware failure, I suggest you to contact the manufacturer or seller for warranty claim, if your dashcam still under warranty.
