You can try the following (no guarantee it will work):
- shut down your player, connect to PC, turn it on again
- this should give you bootloader USB (you didn't mention your player type)
- go to /.rockbox and backup all the database*.* files
- open database_idx.tcd in a hex editor
- change byte at position 0x10 from current commitid (1?) to 20 (the commitid is the 5th 32bit number in the file)
- disconnect and start player