Short connection inside the electromagnet. Re-wind the electromagnet. It is better to use a new spool of wire, but sometimes the old wire may still work. If you need to disassemble the rotor This operation requires quite a bit of strength. Disassemble the rotor with a rocking motion as if trying to break it and at the same time pulling the ends out.
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 electromagnet wires.