What could be an issue that would cause the sound output of an Ibanez brand amplifier to stop working? The headphone output is working, but no sound is coming from the amplifier when the headphones are not connected.
Check the wires on the speaker. Both should be connected. But there won't be sound from it anyway when the cans are plugged in, as you expect. If that's o.k. then the next port of call is the switched socket that the cans go into. There could be a bad connection joining the speaker back up, when the cans jack is removed. Try some switch cleaner, and plug/unplug several times.
It might be caused by one of the internal wires being knocked loose, so that they no longer connect to the speaker. If you are able to take a look at the internal workings you might see an easy fix there - if you are unsure take it to a music shop to have an expert look over it.