Have you tried to boot into safe mode? You can try that. In case it works, you will need to back up the data stored on the drive.
In case it won't work, you will need to use some data recovery tool for DOS mode, so you can try to access the data. Check the Ubuntu Live CD approach out: http/www.tomshardware.co.uk/forum/267999-32-recover-data-mode
You will need a flash drive or a CD, so you can boot (don't install it) Ubuntu. Then you can navigate to the HDD and back up the data.
After that, you can try to run startup repair with your OS CD.