My Fan tachometer keeps dropping to zero. Help

I don't know if this is the right place to post this but I don't know where else.

My Fan tachometer keeps dropping to zero when playing games and then spiking back up to normal. The weird thing is, if I restart my PC and play, its normal. It seems to be happening after I put it to sleep and come back later.

I've had my PC for like 3-4 years and this has never happened.

If anyone know something I would really appreciate it.