master volume knob not functioning

[sugarplumsss]

Under pressure for a possible solution; as an upcoming gig in about 15 hours from now.

The t4 master volume knob,   ( far left , round black knob) is not functioning at all.
Regardless of what setting. the volume is maxed out. The only way to control volume is with the mixer in the tyros. 
I suspect there is a simple solution to why the vol knob is detached from its normal function.

Any suggestions appreciated.

[sugarplumsss]

Haste makes waste. I think I must have plugged into improper outputs.  I was running last minute !   I just tried head phones and volume knob works!!! Sorry for this question.
Thank you
and Happy Holy Days   and holidays too