What is the typical reason midway guns fail? Anyone? I've got the chance to get one of those standup cabinets with the two light guns for extremely cheap but apparently the left gun doesn't work.
You always want to make sure that the monitor face, glass and gun lens are clean. If they're dirty it can cause the gun not to register.
It can also be that one of the gun sensors broke loose of the plug might be loose. No way to tell other than to open up the gun. It's always best to make sure everything is clean first.
As suggested above, crack it open (once you remove it from the game) on a flat surface (the floor is a good spot since it's less far to drop parts). Check the PCB inside it.