Can you boot from a live CD? This would allow you to access the data on your drive to make a backup.

Most likely there is a system file that is corrupted. You will probably wind up doing a restore from the factory hidden partition on the laptop's hard drive. This will overwrite your data and bring your computer back to the way it was when it was first purchased. There should be information on how to conduct a restore in your manual or on the manufacturer's web site.