Yikes, that really sucks. While it's possible to retension a wheel just using the frame and brakes as a makeshift truing stand, if you've never built a wheel before, it's likely that you'll make the problem worse. I would try my luck with a local shop.

But, if you're feeling adventurous, loosen all the spokes so that they're completely slack, and follow this guide starting at "Initial spoke adjustment"

http://www.sheldonbrown.com/wheelbuild.html

Instead of a truing stand, you'll have to use your rear triangle. Loosen the brakes so the loose wheel has room to move, then as it comes into shape, tighten the brakes and use them to check trueness. Again, if you've never done this before you're likely to jack up your wheel. I would still take it to a shop.