Need a lot more information than that.

Brand, specs, age, operating system, how quickly it happens, what are you doing when it happens, etc. etc.

I will say this however, heat can be a major cause of this. If it is an older laptop, dust builds up inside of it. I would suggest dong this as a first step. Take a paperclip and carefully put it into the fan hole to prevent it from spinning and than take your air hose and try to blow out the dust that has accumulated on the internal heat sink. Do it from both directions, through exhaust side first and than through the fan side. Obviously do this when the unit is off.