Sometimes it is simply clogged with dust,pet hair,etc. I would first try cleaning with a can of compressed air available at most stores that sell any type of pc related merchandise. Wal Mart, Best Buy, Staples,etc.
Use the tiny straw that comes with it and blow out all vents ,ports and other openings in the case-thoroughly. Be sure the laptop is shut down, not in sleep mode, and remove the power cord before doing this.
If no joy- replacing the fan is the next step.
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