The break pads have probably swelled due to break fluid getting on them or something else. I had this happen to my car years ago when my dad still had it. Take the break line loose just to see if you get lucky. I wouldn't hold my breath over it though. Most likely you will need to completely loosen the tightener. If you are not familiar with doing that look it up on YouTube. I'm not sure how to explain it well with out getting super wordy. Once it is loose you MIGHT be able to pull the drum easy. Chances are you will either have to get a large 3 jaw puller and pull the crap out of it while hitting with a hammer from the back side or if you can't get a puller you'll just have to beat on the drums with a big hammer or maul.
This is one of the reasons drum breaks suck.