FAN Problem

Hello guys,I own an HD7790. 2GB.And my fan is not spinning.With MSI Afterburner,the fan starts spinning when i set him above 70%.When i restart the computer it spins vert softly after it spins about 2 seconds it stops and i really don`t know why.And as I already told you if i set him above 70% it starts spinning.Help me please!!