Now here is your next step.
Since the pc is working, we can assume the motherboard is functioning correctly.. but to be sure, can you see what RPM you get on the chasis fan(s) and cpu fan in a hardware monitor (bios, speedfan etc)?
after that, take out the fan from the case, and look to see damage on it or smell of burning etc.. try to gently spin it with your finger. observe if it is a free spin or restricted (doesnt want to move) or if it behaves different...
then, if you have access to a 12v source (or whatever the fan is rated for) or a multimeter test the fan with it..
any other info you'd like to add? it may or may not be related to it, such as the environment, hardware/software changes and tweaks you've tried... anything...