DIAGNOSIS This is almost always a jar-to-base connection issue. The blender's safety interlock isn't fully engaged, so the motor gets power but the drive socket isn't turning.
FIX First, unplug it. Grab the blending jar and lift it straight off the base. Look at the black plastic drive socket on the bottom of the jar—make sure it turns freely by hand and isn't jammed with food. Now, place the jar back on the base. You need to line up the shape of the socket with the shape of the motor coupler on the base. Push down firmly and give the jar a slight clockwise twist until you feel and hear a solid 'click'. That's the lock engaging. Plug it back in and try a low speed.
IF THAT DIDN'T WORK If it still hums, the issue might be inside the base. Something might have leaked down and jammed the motor coupler. Unplug it, flip the base over, and check the rubber seal around the coupler for any cracks or sticky residue. If it's gunked up, clean it carefully with a damp cloth. Don't try to disassemble the base.
HEADS UP Never run the motor for more than a few seconds if it's just humming. You can overheat and burn out the motor if the load is locked. If the drive socket on the jar itself feels gritty or won't turn, that assembly might need replacing.