Input jack fell inside - Roland Cube-20x (20w 8" Modeling Guitar Amp)

Join a piece of wire to the connecting wire and solder in place .

Some keyboards use the amp output sockets for headphone use.
You might need some adapters for your headphones.
If it fell in, you would see an empty hole. This would be unlikely because it is held on by two screws. If all fails, buy an amp with a headphone jack input. Plug the keyboard into the amp and use the amps jack.

Make you a hook out of a coat hanger. Get inside and hook the wiring to the jack, take the nut off the jack and pull it up through the F hole. You can plug in to it a patch cord so you can pull it back through once fixed. Check out Guitarelectronics.com for a wiring diagram.

Yep! These poor quality jacks become "unriveted" and break in half. This is a fairly common occurence. Don't be afraid to unscrew stuff. USUALLY the corner protector screws are NOT necessary to take out. OFTEN the screws for the handle need to come out and any other screws from the top and the sides in the vicinity of the amp portion need to come out. Just keep track of where they go. The jack MAY be a part of the circuit board and you will need to get an exact replacement. NOTE: If it is mounted on the circuit board do NOT overtighten the panel nut as that can "de-rivet" the brand new jack !!! They are really poorly made...

Open the unit and restore the jack to the proper place. Do NOT overtighten the input jack mounting nut.

One can overtighten the input jack and "un-rivet" the jack. They are not made very well !!! OFTEN when they appear loose, they HAVE un-riveted the threaded sleeve from the metal inside the jack... that USUALLY requires REPLACEMENT of the jack itself.

The cabinet backs on a lot of these have to be PRIED out as the Tolex covering often makes them tight.

The amp portion is often retained by screws from the top and or sides AND even the two holding the handle on... there are many different configurations. Sometimes the amp comes out the front, sometimes the back. It is easy to shear off the speaker wires when removing some of them.

I think I know what happened and since I am not in front of the unit I will go over couple of things and a general cure.
You have a case of signal ground uncoupled from chassis ground and your body is a pickup of electrostatic charge, this charge is present every where and can be up to several thousand volts but at almost nil amps. Since we are in a power grid this charge has a 60 cycle component and this overlays the the charge. That is the tutorial for the theory.
The change of the jack has made a disconnection of the signal ground and chassis ground and this needs to restored to get rid of the hum. It might be something as simple as paint or a fiber washer preventing the connection. Also the wiring may have to be reversed on the hot side. If that fails you may need to add a coupling device and since a picture is worth a thousand words, here is a diagram and I hope it works here....

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Try to see if you can fish one of the wires attached to the jack out enough to get the jack through the F hole. If you can get to the jack, get some medical tubing ( like they use in sling shots for example ) that will fit inside a jack. Stick the medical tubing through the guitar starting at the outside of the hole the jack is supposed to be in. Once you can fish the end of the tubing out through the Fhole, insert it into the jack and gently and carefully manuever the jack back into position by pulling on the other end of the tubing. You should be able to slide the washer and then the nut down over the tubing until you can thread them back into -place. Hope this helps.

If you can unsolder the jack, take it to any good electronics supplier and they should be able to sell you a replacement. The manufacturer is not as important as the pin configuration and the height it stands from the surface of the circuit board.

The safest way to fix this is to take the amp apart to get at the jack. If the jack is a solid black plastic shape, then you should replace the jack completely. Otherwise you should have access from the side of the jack to remove the piece of hte plug that broke off.

Dan