Re-wind the electromagnet. It is better to use a new spool of wire, but sometimes the old wire may still work. Usually occurs with 2 magnet rotors. The rotor will not start spinning on its own. Give it a slight push.
Rotor is jammed.
Find the exact cause and fix the problem. This may require breaking off the stands and reattaching them to the board. You may try to lubricate the axles with WD-40 or any other lubricant. Electromagnet attracts the magnets instead of repelling them. Switch the battery connection wires.