If it is the same as a bicycle, you would use a chain tool, not a hammer.
The chain tool may be able to remove the bad link and reconnect the chain
together. Seems odd though that the chain broke. Are you sure your
springs are adjusted correctly so that the opener only guides the door and
doesn't need to move the full weight of the door.
If you disengage the opener from the door, is it very heavey to lift? If
the springs are set correctly, you should be able to open/close the door
with one hand without straining yourself. If the door won't stay down,
then the springs are too tight. If too heavey to open, then they are too
loose. Adjusting the springs will depend on what type they are. It can
be a dangerous task so be careful (or hire an expert, once you've
determined that they are not in correct adjustment).
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