Well, on the assumption that your audio output is working okay and it's just the meters, a couple of things to look at if you're confident enough to open up the mixer...
First of, there's permanent links from the main outputs to the meters--these are permanently there except when over-ridden for PFL etc. Have a check that there's not a break in that circuit.
Second, there are switches to turn off the main outs and feed PFL to the meters. The switches are driven by a voltage from the PFLs on each channel. It's worth measuring to see if that voltage has somehow been jammed on even with nothing PFL'd...or if the switches themselves are somehow jammed.
The block diagram at the back of the operating manual will give you some clues although a full service manual would be more fun. FYI, the service people at Allen and Heath in the UK are really friendly and helpful. It's probably worth ringing them to see if they can talk you through a fix--no problem if you're in the UK or use Skype or similar if you're in the US since it'll be cheaper than a normal call.
