"pistons don't get "stuck." it's either seized, or a bearing's destroyed, or something else that's probably not repairable. you can take the 4 nuts off the head and look inside, or pull it apart. it's pretty simple."
Pretty much what I was thinking with the possibility that if the engine was running before it went to the shop and the jug was removed maybe the repairman didn't know to align the rings before re-assembly.