I have an old Shelton Jet Vac, Series 300-P shop vac that I used once yesterday and when I went to turn it back on it didn't do anything. No odd noises or anything when I first used it. I checked power cord, switch and made sure all connections were tight and everything is ok. Beyond that,I am not sure if there is anything else I can check with a multi-meter and realize this is a very old machine so that parts may no longer be available to repair it. Also, after finding this site I guess that there are others with the same machines still being used. Any information that you could provide would be greatly appreciated. Thanks very much!
Sometimes when I suspect the brushes have worn all the way down, I'll turn the unit on and twist or jar it sharply -- if it starts, it's likely your carbon brushes are worn down to nothing and need to be replaced. This takes many, many hours of use, so if the thing has been gently used, that's probably not it.
Multimeter troubleshooting: you can do this with the cleaner unplugged using the ohms setting to check for continuity through the cord and switch (one conductor at a time), or you can use the volts setting and have the cleaner plugged in as you test across hot and neutral at the cord, then the switch. If the motor leads are getting 120 volts, then your motor is dead. Anything besides brushes is pretty much non-repairable unless you're really determined. Best in that case to find a working motor to install in the unit.