Try Start-up repair first. If that doesn't work, use System Restore to rollback a few days.
As a last resort, you also have the option of completely restoring the machine back to factory settings. This will wipe everything, so any important files will need to be backed up first.
Comments
What exactly happens when you try the different options?
Were there any prior errors, upgrades, Windows Updates, etc.?
Can you use F8 to access the Start-up repair menu? Follow the instructions here:
http://windows.microsoft.com/en-gb/windows7/what-are-the-system-recovery-options-in-windows-7
Try Start-up repair first. If that doesn't work, use System Restore to rollback a few days.
As a last resort, you also have the option of completely restoring the machine back to factory settings. This will wipe everything, so any important files will need to be backed up first.