What is wrong with my accordion?

i recently got an accordion from an estate sale. it seems to be in great condition although i'm just learning how to play it and i don't know a lot about them. after playing it a few times the bellows is stuck OUT and it won't push back in on the bottom part of the bellows-it's just hard like it froze up or is caught on something and won't budge. what is wrong with it and what will it take to fix it?