Black and decker mm575 worked fine but now the motor tries to start but locks up and just hums. Any idea

Before you start removing covers and panels unplug the mower. Turn the mower over and carefully try to spin the blade by hand. Your checking to where the issue is most likely to be.
Does the motor sound different when you pull the Start trigger? If it sounds a lot faster than normal, it could be an issue between the motor and gearbox.
Does the blade spin really easily and feel like it's not bolted down? Does the blade not turn at all like it's 'locked' or 'jammed'?
- A loose or floppy blade is commonly the centre mount bolt for the blade is loose or the blade isn't seated correctly.
- A 'locked' blade could be a gearbox or a motor issue.

http://www.kmart.com/black-decker-side-discharge-grass-bag/p-9990000032185411P Try here.

Take off the black plastic hood (4 screws), clean debris and check connections to, and condition of, shunted brushes. They're available from B&D parts for a few bucks.

while unplugged rotate the blade a few spins to move the brushes if there stuck make sure the outlet is good and try another power cord

does it have gas?

Hi Kevin,

User a pair of channel-locks to grab the square spacer below the nut. Then tighten down the nut using wrench.

probably a loose or out of balance blade

Two things come to mind. The brushes in the motor should be replaced. (Get them at a local motor repair shop, but take one of them in with you to get the correct one). If that is not the problem it may be the switch in the handle that you turn it on and off with. The switch may be corroded or bent. Good luck...Lee

Sounds as though you either broke or badly damaged the flywheel key. It is located on the shaft that comes up through the flywheel. If you have ever taken the cowling off, you know how. Just make sure to use the proper size and shape flywheel key. This key is designed to break in order to save the more valuable parts of the machine. Good luck...Lee

see www.toolpartsdirect.com .look for black & decker icon.
u have to enter model #