How do you remove a cello endpin that has slipped inside the cello? Most likely the endpin's collar has fallen off. A lot of the newer Chinese endpins do not have good threads that holdd the knurled collar securly. Here are the steps that I use to remove endpins.
Loosen the strings
Remove the bridge
Remove the strings from the tailpiece
Remove the tailpiece
Remove the end pin plug. Since they are a taper fit, they usually come out with a slight twist.
Position the end pin so that it's parellel to the F hole on the "C" side.
Going through the "C" side F hole, use a pinching style grabber to pick up the end pin. The balance needs to be a little lighter on the end pin hole side.
Then pick up the endpin and adjust the grabber so that the point of the endpin goes into the end pin hole. You may need to play around with your balance a little to get it just right. Then grab the endpin with your fingers of needle nose plyers and release the grabber and pull the endpin out.
Most of the Chinese endpins that have this problem use a M8 1.5 pitch. It's difficult to find a knurled collar but you can use a nylon locking nut or just simply a regular nut.
Some endpins have a little stop so you can't take the endpin out. Most modern endpins are quite long so you don't really need the stop. I usually remove the stops because if you try to sharpen an endpin that you can not remove from the cello it's somewhat difficult.
Put the endpin in a vice so that it's very secure.
Use the red locktite threadlocker, then screw the collar or nut on as tightly as possible.
Put the endpin plug back into the cello.
Slip the endpin rod back into the plug.
Install the tailpiece and hook the strings back onto the tailpiece.
Set up the bridge
Tune and make sure you keep the bridge straignt while you tune.