DIAGNOSIS The motor is getting power (hence the hum) but it's not turning the agitator. This usually means the drive belt has slipped off its pulleys or broken entirely.
FIX 1) Unplug the machine. 2) Remove the two screws at the top of the back panel and slide the panel up and off. 3) Look at the large black rubber belt. It should be looped around the motor pulley (small) and the drum pulley (large). If it's off to the side or in pieces, you found the issue. 4) If it's just loose, you can often re-route it. If it's cracked, glazed, or missing teeth, you need a new one. Note the routing before you remove the old belt.
IF THAT DIDN'T WORK If the belt is intact and properly seated, the hum could indicate a seized motor or a failed motor capacitor. The capacitor gives the motor the extra jolt to start turning. A bulging or leaking capacitor is a clear sign.
HEADS UP When working behind the washer, be mindful of the water hoses and electrical connections. It's a tight space. A belt should have about 1/2 inch of deflection when you press on it. Too tight or too loose will cause problems.