The good news is your data is probably OK; it's just that you can't get to it. Unless of course your hard drive has died. In that case maybe you really should have backed up your data :>)
Assuming your hard drive is OK, you can access your data and copy it onto removable media or another PC using a bootable rescue CD. With a bit of luck you may even be able to use the rescue CD to fix the problem with your PC and get Windows to boot again.
A rescue CD works by allowing you to launch an operating system directly from the CD so you don't need Windows to be running on your PC. From that CD-based operating system you can then access your hard drive and all your files.
There are lots of rescue CDs available based on different disk-based operating systems. Some use DOS or a DOS clone such as FreeDOS. Others use Linux, but my favorites CDs make use of a cut down version of Windows called the Windows pre-installation environment, the best known of which is Bart's PE .
Of the half dozen or so rescue CDs I have in my tool kit, the one I use most frequently is UBCD4Win . It's a Windows-based CD that uses Bart's PE Builder to create the operating system. What I like about it is the huge range of tools included. Yes, you can set up a Bart's PE disk yourself with the same tools or even more, but it's not a simple process, while UBCD4Win comes ready to go.
One of the many valuable tools on the CD is the free imaging program, DriveImageXML, that allows users to create and restore images of their disk drives.
It is particularly valuable because it allows you restore an image of the drive containing Windows even if your Windows system is not booting. For this to work, though, you must have created an image before your system failed. That's a topic I covered in this month's editorial.
You can even setup UBCD4Win to boot from a flash drive, providing of course that your computer's BIOS supports USB drive booting. Personally I think you are better off with a CD. CD booting is supported by all but the most ancient PCs and a CD is only marginally less convenient to carry than a flash drive.
By far the easiest way to create a bootable UBCD4Win rescue CD is to order the preparation CD from the UBCD4Win website . It contains everything you need except the Windows installation CD that came with your PC. Note that the preparation CD is not a complete ready-to-go boot CD but rather the components you need to build one. However, full instructions are included. At $5.95 for US customers and $7.95 International it's a bargain. Hey, you even get a free CD burner program thrown in!
If you are too impatient to wait for the CD to arrive by snail-mail or too stingy to pay the $5.95 then you can download the various files from the UBCD4Win website . The instructions on the website are excellent. Be aware, though, that the download is around 230 MB and the boot CD building requires around 2.5 GB of free disk space.
If files that big sound too daunting then try one of the many Linux based rescue disks . Many are smaller and simpler to set up; just download the ISO file and burn it to a CD. The popular Ultimate Boot CD  for example is an 87 MB download. Usage though can be a little daunting for those not used to the Linux environment but there's an excellent Linux rescue disk guide for Windows users here .
Whatever option you choose, do take the trouble to set up a bootable rescue CD. One day you will need it. This, I can assure you.