You get behind the board, turn up the volume, and nothing happens.

All the lights are on – what’s wrong?

In order – something is unplugged, turned off, or a battery is dead. Usually broken or faulty cables cause bad noises before they quit altogether.

First – use your headphones and listen to the channel. Check master channel gain, mute, signal routing, group gain and board gain to make sure it’s not something on the board. Then start at either the source (instrument/microphone) or destination (speaker) and trace the signal path. If the cables are connected all the way to the equipment then check to see if the equipment is on (yes this still happens to me).

Microphone/Instrument: 
Does the microphone have a switch? Wireless mics often have a power switch and a mute switch – both must be on and the battery must be good.

Is the microphone a condenser mic? If so it needs phantom power (or a battery).  Make sure you mute the channel before turning on or off phantom power or change a cable. The loud pop can hurt ears and destroy equipment.

Are there active direct boxes in the line? Many direct boxes have a preamp built in and either use phantom power or a battery (label these so people know it). Many guitars and basses have batteries as well though usually the musician is used to dead batteries and will try that right away.

If all this fails start looking at peripheral equipment. Usually it is all turned on at the same time as the mixing console, but an effects box or compressor can interrupt a signal. If it is unplugged or turned off it might stop the signal.

Try a different channel on the board. I have seen bad channels but they are rare.

Try a different mic/instrument. I have only once seen a standard wired mic die. Of course it quit during a live show and I had to wait for a break to replace it.