Sounds like you don't have a backup, so if you can afford one go and order yourself a USB hard drive asap!
Shut down the machine, start it up again and keep tapping F8 (before you see Windows loading) and you'll get the option to boot up in safe mode.
If that works, you'll have an opportunity to back up your files in that state. If safe mode boots up ok then either corruption, buggy software, malware, or a dodgy driver is at fault. If safe mode didn't even boot then corruption is most likely.
You can try the built in repair options and if that doesn't cut it then you're probably looking at a reinstall, unless you can take it to someone who knows how to dig deeper into the cause.
If you can't boot up to get your files off, you'll need to attach the drive to another machine in order to do so, most likely with a USB adaptor that you can pick up from eBay fairly cheap.