DIAGNOSIS Sticky smoothie residue (especially from sugary or dairy-based drinks) has seeped under the rubberized button membrane on the motor base, causing it to adhere and not spring back up.
FIX Unplug the unit. Dampen a cloth with warm, soapy water and vigorously wipe around and over the stuck button, pressing the cloth into the edges to work soap underneath. Then, press the button repeatedly while the area is still damp to work the solution in. Dry thoroughly. Isopropyl alcohol on a Q-tip can also help dissolve sticky residue.
IF THAT DIDN'T WORK If the button physically feels like it's catching on something, the internal plastic mechanism might be broken. That's not really fixable without opening the base, which isn't recommended.
HEADS UP Clean the button area immediately after making a smoothie, especially if you've had a spill or overfill. Don't let sugary liquids dry on the controls. A quick wipe prevents this whole problem.