Try this:
1) Put the calc into 'maintenance mode'
This can be done by pressing [DOC][EE][ENTER], and then, while holding these keys, use a pen to press the Reset button on the back of the handheld.
2) Choose option 4 to wipe the calc completely. MAKE SURE YOU HAVE SAVED ANYTHING YOU NEED!!!!!!
3) Plug the calc back into the computer
4) open the ti nspire software and it will prompt you to reinstall the OS. Drag your preferred OS onto the calc from your documents.
5) Try it then, and set everything back up
If this dosent work, try contacting TI.
Chris