same thing happened to me not long ago. Most likely it is due to some app i installed. Have you been installing any apps from extras-testing or extras-devel? If so one of them might have broke it. I can't be for sure which one broke mine but i have a suspicion.

If you have been making backups you might be able to go to a backed up version where you didn't have this issue. Otherwise you are looking at reflashing your device. Check this link for a how to:

http://thenokiaguide.com/2010/01/11/...sets-all-data/

there's also a wiki here @ maemo.org but i am not sure where at at the moment.