My a/c was working fine yesterday, all of the sudden stopped working, I can hear a humming sound from the fan, and it would start if a rotate the fan, but the compressor won't start at all , some guy from the gas company told me that it might a selenoid wiring gone bad , this selenoid is responsible for kicking the compressor in, if the selenoid breaks it might not send enough voltage to the compressor to start, would this be the problem? Thanks !

Look into the capicitor being bad,, sometimes the top will swell up ,looks like a frosen coke can,should be flat,if so replace with an exact match after you have turned off breaker and shorted the posts together with a screwdriver