If you can get to safe mode, it's good.
I'm assuming it happened recent. If so, while in safe mode, type Start>Run>msconfig . There is a button marked System Restore. Click it, choose an earlier date (before this hapened) and let it do the restore, and it will restart. Try getting in your user account now.
If that's not working, and if it is a corrupt user profile, while you're in safe mode, make a new user account.
Restart and try to enter the new user.
If it works, then U may need to copy all your old Desktop and My Documents to this new user
Old desktop and docs are in C:\Documents and settings\Your_old_user_name