Diagnosis
A drain pump that won't stop humming means it's trying to pump but can't move water. This is almost always a blockage in the drain hose or a stuck check valve.
How to Fix It
First, check the drain hose under your sink. Make sure it's not kinked or squashed where it connects to the sink waste or garbage disposal. Then, disconnect the hose from the sink connection and lower it into a bucket. If water flows out, the blockage is in your sink plumbing. If not, the blockage is in the hose or the dishwasher's internal pump. You can try to clear the hose with a long, flexible bottle brush.
If That Didn't Work
The blockage might be in the pump itself. You'll need to access the drain pump, which is behind the kick plate at the bottom front of the machine. Turn off power and water first. The pump has a small impeller cover you can often remove to clear debris like food particles or glass fragments.
Heads Up
If the pump is humming but not pumping, don't let it run for more than a minute or two. It can overheat and burn out the motor. Turn off the power at the breaker to stop it immediately.