So now that I have my Trusty Triumph, about 2 miles into my 2.1 mile trip to my first bike maintenance class the pedals stop moving.

Looks like the chainring has moved over and is now on top of the the chain stay... there must have been play in the crankset that I didn't notice when I bought it.

Going back to the shop to work on it myself tonight, and they have volunteer mechanics to help out, but I have no idea what I'm doing. The mechanic I talked to at class last night seemed to think it had been put back together improperly and once we reset it, it won't do this. Have done a cursory search online but I'm not sure what to call it and this may not be a common problem.

Sorry no pictures, I left the bike at the shop since I couldn't actually ride it.

Any advice?