Swap the cpu fan for a case fan. See if you still get the error. That error happens when the motherboard detects nothing/no rpm on the cpu_fan header, and prevents the pc from turning on and damaging the cpu which can hit 250°C+ in less than 3 seconds.
Most likely the bearings in the cpu fan motor are toast, requiring a new fan, or a full heatsink replacement, whichever ends up cheaper and/or better.