The headphones don't work or the speakers don't work?
On purpose they design a circuit to detect when the headphone jack is in use, so it mutes the output to the speakers. Sometimes pulling the jack halfway out can let you have both output in headphone and speaker. Although what you hear in your headphones may now be in mono, or on one side. Tip, Ring, Sleeve or TRS.
If you need to put the jack in half way for the headphones to work properly then your jack is broken on the amp or powered speaker. Possibly a solder contact on the jack to the circuit board broke. Making the jack TS or TR or RS. That would explain why you would need to put the headphone plug halfway into the jack to hear audio.
