DIAGNOSIS The motor is encountering a hard mechanical jam. The blade assembly is seized, likely by a frozen ingredient or a piece of fruit/vegetable core caught between the blade and cup base.
FIX Remove the blending cup from the base. Manually rotate the blade assembly inside the cup. If it spins freely, the jam is in the motor base. If it does not spin, disassemble the cup. Remove the rubber gasket and blade. Clear any fibrous debris (pineapple core, mango pit fragments) from the blade shaft. Reassemble securely. Do not use the blender with the jam present.
IF THAT DIDN'T WORK The motor base internal drive coupling may be stripped. This is visible as worn, rounded plastic teeth on the spindle that engages the cup. This requires replacement of the base motor unit.
HEADS UP The 900W motor has an overload protection circuit. It will cut power after 2-3 seconds of a hard jam. Allow it to cool for 10 minutes before attempting to clear the blockage and retry.