Pedals won't turn forward

[uvachic]

okay, so i have a 1982 Schwinn Varsity 10 speed, 27 in wheels that i picked up used from Oscar Wastyn Bike Shop in Chicago. I took it for a short ride around the corner when i first got it and it rode fine (they tune them up and repair them before selling used bikes) Today, I pulled it out of the basement to put on the bell and some other accessories that i had bought. the front tire was a little flat so i pumped it up but caused the tire to become improperly seated. So I removed the front wheel and took care of that problem. I reattached the wheel and Afterwards, I tried to take it for a ride. I started to go but the the back wheel stopped turning after I shifted gears. When I pedal it backwards it goes just fine but when I pedal forwards it's like the kids bikes that have breaks on them when you pedal backwards. I don't know what happened to it, it didn't fall or anything while I was fixing the wheel. Anyone have any ideas? I would appreciate the help