|Do you have an external hard drive plugged in ?|
If so, try unplugging that then attempting to boot.
(I know from experience that Vista can fail to load all the way when an external drive is plugged in for reasons that are NOT clear to me.)
If that doesn't apply or if it does but that doesn't help....
What went on the last time you used the computer when it still worked fine before you shut down the computer?
Did you load software, or did Windows Auto Update load updates while shutting down, or did some program auto load updates or load updates with your consent ?
Try booting into Safe mode, or Safe mode with networking, or loading Last known good configuration.
Remove any bootable CDs or DVDs in drives.
Press F8 repeatedly while booting, do not hold down the key, starting early in the boot sequence.
When the boot choices menu appears, choose
Safe mode, or Safe mode with networking, or load Last known good configuration.
If that works, click on the Vista logo button (Start) bottom left of the desktop screen, in the Start Search box, type: cmd , click on OK,
then type: CHKDSK /F C: ,press Enter
Answer Yes to any question. You will probably be told CHKDSK will run the next time you boot the computer. Restart the computer, let it boot normally, and let CHKDSK run.