Check to see if the action block lever, the button at the rear of the trigger guard, is functioning.
Pushing it up should release the action and gun should pump.
Also check the safety button on the top of the receiver to see if it works.
If it's broken, it could be jamming that same action block lever.
It's possible that it's only a stuck case in the chamber.
Give it a robust pump, maybe, to eject.
Is the round in the barrel chamber live or not?
Are there live rounds still in the mag tube?
Lock the doors, they're coming in the windows.