MH3184XPS-3 started humming last night. I unplugged it. I later plugged it in and it was still humming. There is power because the clock worked, but the microwave would not start.

You should keep the oven unplugged until it is fixed or unless it is being tested.

I would first check to see if the oven is heating when it should not be.

You can place a cup of water in the oven, close the door, and let it set for about 30 seconds. (No longer or it may be damaged from overheating).

If it is, then I would suspect door switch trouble (see this text file) or an issue on the control circuit board, which we regularly repair with our $39.95 repair service.

For more information on door switches, see these help files:
http://031d26d.namesecurehost.com/mwd/safety.txt
http://031d26d.namesecurehost.com/mwd/disassembly.txt
http://031d26d.namesecurehost.com/mwd/doorsw.txt

Once you've checked it out, if you need further help, feel free to contact us here.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.