If the board's still up, try the following
first off, process of elimination: can you bypass the console and confirm that the monitor system is ok?
second: if the power supply visually looks ok, and nothing smells like burning, then let's assume that it's board related:
the bus meters indicate level? what kind of level? tone? program?
try the solo bus with a channel or 2: same problem?
what you want to do is try to trace the source of the problem:
could be something big, or could be something small, like a component in the master section: chips / caps / opamps are the common culprits
are you alone? can you get someone else to either hop online and hit some sites - ddr would be a start, or make some local calls to techs in the area, or find a national service number for the distributor / manufacturer
With luck, it's either relatively minor, or a known issue that has a fix.
meantime - what's the session for? can you work around the problem if you can? sounds strange, but depending on console, it's possible to bypass the master section with some clever patching if you have subs that are working and can route straight to monitors: if you don't need headphone mixes or aren't doing a full-on mixing session, just o'dubs, this might work.
And remember - you're in showbiz - and the show must go on!
Don't panic, don't give up, and start thinking OTB for solutions; you still have time to troubleshoot and possibly fix the problem, even if the client shows up, they'll usually give you a bit of a window before packing it in.