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I'd imagine in the 'things that can go wrong' area we are either talking about a powered mixer or else a mixer/power amp combination. It's unlikely two speakers would stop working at exactly the same time.

There's a limited amount of fault finding you can do. Ideally you would have a known working counterpart to all the equipment so that you can quickly narrow down which component is at fault.

Main culprits are blown fuses - mains leads and in the equipment itself. Check the plugs for loose wires. If they always use the same multi-way mains block then make sure that is working as well. Do the power lights come on?

Some power amps have electronic overloads that can trip and need a manual reset. These are normally on the back panel, so try pressing any reset buttons you can see.

Apart from fuses, the only things that you can easily check are any in-line jack sockets on the main bus - especially on mixer amps. These often have 'mixer out' and 'power amp in' connections and if they don't get used, the connections can tarnish and the signal continuity gets broken. Again it's unlikely that both channels would suddenly go at the same time on a stereo mixer, but possible if it's only a mono unit. Inserting a link cable between the 'out' and 'in' sockets can ofter be used as a temporary fix for this problem (use a squirt of DeOxit or Servisol contact cleaner as well. Replacing the jacks is the long term solution here.

If it's a mixer with an external power supply, then one of the voltage rails could have failed, so you may see power-on lights but the other voltage(s) required may not be present. It may be due to a damaged cable or to the PSU itself failing. You shoud be able to check with a multi-meter if you've got the required voltages on the end of the PSU plug.

Failing that, if you feel like opening it up you may be able to spot an internal fuse that has blown. If you spot
some burnt out circuitry then you've found the problem but its probably beyond your ability to rectify.

Before you take anything apart, (apart from diconnecting the mains and waiting an hour or so to ensure any power supply capacitors have discharged to avoid the risk of shock) check the age of the kit. If its still in warranty, then back to the shop. If it's not in warranty, then someone will have to make a decision as to whether it's worth repairing or buying something new. They can try flogging the old one on eBay 'for parts or repair'.