thats quite strange, a turntable will send audio to the mixer if its pluged in or not so it points towards something being loose or worn out within the turntable.

(that is if i have understood what u mean)

have u tried the turntable on another mixer channel, did the sound come back on when u turned the deck on then off or the mixer on then off, its important to make sure which unit is causing the problem.

check the phono cables from the turntable that is causing problems, if there not removable (i.e technics etc) make sure that it hasnt been yanked loose inside, if it keeps happening send them off to the manurfacturer or contact the place of purchase.

just to be safe swap the headshells on the decks too incase it lies in a faulty headshell