The original rear wheel probably got damaged. It may be best to find another rear wheel that will take an 8 speed cassette. You'll also need to check the spacing between the rear dropouts and get the wheel to match.
I recently put an eight speed cassette on a ten-speed wheel I got from Craigslist with no problem at all.
If you find a wheel, you'll also need to adjust the derailleur and cable to make it work.
Is there a bike co-op in your area? That would be a good place to start, for both the parts and a mechanic to help out.