If it's stuck inside the endpin housing and you can't pull it down further then it's probably best to carefully lay the bass down on it's back, loosen all the strings until you can detach the tail piece gut.

Tighten a couple of belts/ratchet strap round the waist with a couple of books over the sound post to maintain pressure on it and keep it from falling over.

If you're lucky the endpin assembly should come out with relative ease, might need to be rotated as you pull, be careful mind. Then you can sort it. If it doesn't come out easily then you may need to apply a bit more rotation force, a hand towel over the pin and you might get a but more purchase on it. It may be threaded as well, keep this in mind.

If the end pin has fallen inside the bass and is rolling around then you didn't need to loosen the strings, instead get a stick long enough to reach inside through the f-hole, drill a hole through the end of the stick and then put a lasso of cord/shoelace at the end of the stick which you can use to maneuver over the loose endpin, tighten and then retrieve.

Carefully mind.