You'll need to give us a little more information please..

Operating system? Desktop or laptop? Brand and model number? Age of the computer?

What steps have you tried so far to stop the problem?

When was the last time you opened the computer case and used compressed air to blow out all the dust from around the processor, heatsink, the power supply, all vents and all fans?

Does the fan problem happen all the time or only when you play games or view videos?

Hope this helps and let us know more.