Silent Channel

Ruled out so far:

Check the crossfader

• Move the crossfader fully away from your channel, then back.
• Find the crossfader ASSIGN for that channel (A / B / THRU). Set it to THRU.

No ASSIGN switch anywhere? Some mixers have no crossfader at all (DJM-V10-LF, DJM-V5, rotary mixers). On those every channel already behaves as THRU, so this is not your problem. Keep going.

Is the sound back?

You follow these steps at your own risk. Full disclaimer.