The same sort of thing happened to me 2 months ago. Vista is often updated by Microsoft and if an update requires a reboot the update will finish during boot-up, as you have seen. I guess sometimes the update goes bad and prevents the computer from ever starting.
I emailed my computer vendor [HP] support people.
They recommended starting in Safe Mode and doing a system restore:
“Go to Start - All Programs - Accessories - System Tools - System Restore.
OR
Press the WinKey + R to open the Run dialog box and type or paste rstrui.exe then press enter.”
If that didn’t work they said to go into System Recovery mode and initiate a Startup Repair. This procedure may vary according to your manufacturer.
Neither of these worked for me so I emailed Microsoft, who replied that because Vista was preloaded by HP, HP was responsible for fixing it. To make a long story short, Microsoft was nice enough to mail me a Vista Recovery Disk, which helped me restart the computer. [I can’t remember the exact steps.] It turned out Vista had never made any restore points after all, and would only boot by using the Recovery Disk after that, but at least I could recover my data and settings.
So, email your computer manufacturer for support. If you have a recovery disk, it might get you running again. But don’t use it without instructions; it could wipe out your data.