DIAGNOSIS That's a classic sign of a jam. The motor is trying, but something's physically preventing the blade assembly from turning. It's usually a piece of food (like a tough vegetable stem or a date pit) wedged between a blade and the container wall.
FIX First, unplug the blender. You need to manually free the blades. Take the container off the base. Grab the blade assembly's central coupler with one hand and the container with the other. Try to rotate the blade assembly counter-clockwise (lefty-loosey). Don't force it hard, but a firm turn should dislodge whatever's stuck. Once it moves freely, check inside for the culprit, remove it, reassemble, and try again on a low speed.
IF THAT DIDN'T WORK If the blade assembly is completely frozen and won't budge, the container might be damaged. Inspect the plastic container near the blade for a hairline crack or a deep gouge that's warping things. A damaged container can pinch the blade assembly. You'll likely need a replacement container.
HEADS UP Always start the blender on the lowest speed (Variable 1) and quickly ramp up to high. Starting on high with a thick mixture is the fastest way to cause this jam. Think of it like easing a car into gear, not dropping the clutch.