The circuit breaker for the disposal is tripped. At least check it and make sure you’ve got 120v to the disposal before you freak out. And how do we check for voltage? Can we look at it and see that it has voltage? Nyet, comrade, we must measure for voltage using our voltmeters. Look, if you’re not willing or able to do at least that, you shouldn’t be fooling with this stuff. Really.
The disposal’s internal circuit breaker is tripped. Look for a little red button underneath disposal and press it in. If it was out, you’ll hear it click when you press it.
It hums but it don’t grind.
Your grinding wheel is jammed. Use the wrench that came with it (or use a 1/4" allen wrench) in the socket underneath and wriggle the snot out of that thang. You may need to reset the circuit breaker, too. If the disposal sounds rough after running for several minutes, then you’re on borrowed time and you’ll need to replace it soon.
It runs and seems to work but it’s noisier than my mother-in-law.
A piece of metal went down the disposal. It may eventually work itself out.
The main bearings are wearing out and disposal will need to be replaced soon.