My bike's back tire just jammed. Help!

I was riding my bike today, everything was working fine, but all of the sudden the back tire just jammed.
I checked it out, and it seems like the gear got stuck somehow, like maybe one of the screws got too tight. When i try to peddle, the gear nor the pedal move. When you look at the bike, the it seems as if there is a lot of extra chain (slack). I am trying to fix it right now, but I am definitely not a bike guru.
Any suggestions of what I can do? (don’t want to take it to the shop if I don’t need to)