What was the last event that occurred in your Windows? Do you remember?
Follow step by step this way.
The First step:
When turn on the device, Change your first boot device priority to your optical drive (CD/DVD ROM). Insert the Windows vista installation disc into the optical drive.
Your computer try to boot from the Windows Vista installation disc.
The Second Step:
When your system boot up from DVD, Click the 'Repair your computer' option like as picture.
The Third Step:
Click the 'Startup Repair' option.
The Fourth Step:
Let the startup repair wizard do its thing. When it's finished, it should prompt you to reboot, and then the process is complete!
The Fifth Step:
If repairs are not successful, you'll see a summary of the problem and links to contact information for support. Your computer manufacturer might include additional assistance information.
I think this way is your answer and