You should have a Windows Vista Disc that came with the computer. If not, there's probably a system restore disk. When you put that disc in, restart the computer and when it's first starting up, you should see "press any key to boot from cd"
When you see that, press any key so it runs the Windows Vista setup disc. Then, select options like you're going to install vista and you'll see the option for "Repair Vista Installation" or something like that. Choose that option and then choose the automatic method. It will scan for problems that could prevent it from starting and hopefully find/fix them. When it's done, it should ask you to reboot your computer. Go ahead and do so and then remove the disc from DVD/CD Drive. After that, with any luck, your computer should start up.
Let me know if this works