Might be the fans are clogged up with dust and are having a hard time cooling the system.
Depending on the model, you could be able to take off part of the bottom lid covering the fans, or the keyboard revealing a little hole on the heatsink.
If you're in luck and can unmount those, use a can of compressed air to give it a clean. If you can't unmount anything, it'll not help too much (on the long run, at least), but you could try compressed air anyway and blow it inside the holes on the back/bottom/side of the laptop.
If that doesn't fix it, you might need the help of a professional (unless you want to do it yourself) and open it up for a more thorough cleanup.