Try rebooting to tap F8 repeatedly, see if Repair console is on the Advanced Boot Tools menu.
If so run Startup Repair which automates all the tests and fixes. See if that will start it. If not try System Restore from the System Recovery Options
If not, I would write the Windows 7 DVD ISO to flash stick: Install UltraISO, on File tab open the ISO, on BOotable tab Write Disk Image, Format, Write. Boot under USB or HD"s using one-time BIOS BOot Menu key. Now maybe the Repair will work better. If not try System Restore to an earlier point.
If it starts and still has performance problems, here are tips for troubleshooting to get to the problems, including if necessary running a Repair Install
from the flash stick or from the extracted ISO files.