Hi there, you referring to exhaust fans? If they no longer work then you probably need to replace them, if you have warranty you can send it in to Dell to fix. If not you will need to buy the fans and install them yourself or take it to a computer shop.
Okay then you will just need to bring it in to the place you bought the laptop and explain to them the defective fans that no longer work. They should replace it for free, if you ordered this then they may ask you to ship it back to them.