Hi annhars,
I have had this happen to my PB360 several times (I only wish I knew why).
If your PB is still working you can recover by selecting [Settings] - [Maintenance] - [Format internal memory].
It sort-of restores you back to a "plain vanilla" system.
Your books on the internal memory will be gone. Those on your SD card should be OK, but I always remove the SD card before I start the reformat.
You will also lose anything you added manually to the /system directory e.g fonts, dictionaries, logos, themes, fbreader180, extra games.
Once the reformat is complete you should end up in the main menu - possibly in German - so you may need to go into [Settings] and reset to your chosen language.
At this point I always switch the PB360 off and reboot it.
You can then put back your manual items as mentioned above (fonts, dictionaries etc).
I then do another reboot before I start to put back any books and reset all my personal preferences in Settings - like key mappings.
Hope this helps
