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I am assuming nothing has changed with the unit's plumbing set-up re: a new garbage disposal etc.

Basically you are checking the pump now.
Since you can hear it humming you know that it is getting power.
Since it gets power but does not pump that can only be three causes.
1. A plug is the drain system stopping the water.
2. Something jammed in the pump's impeller
3. The pump is shot

Check that the sump is clean.
Check that the drain is clear from the pump to the house plumbing.
You could remove the drain hose from the house plumbing and drain it into a pail to eliminate the house plumbing as the cause. Just be sure to hold the hose securely in the pail and put a towell over it so it does not splash all over.
Check the flapper valve (Item 328 in Section 3)
Remove the pump and check it.
If all OK I would replace the pump.