Running a laptop with a fan that cannot turn on could lead to catastrophic hardware damage. A functioning cooling fan for a high heat-producing laptop is as essential to the device's longevity as food and water are to a human. If you're a novice, you can try non-invasive fan cleaning to fix the system, but the more complicated fixes require professional help. Note that some ultra-thin laptops and hybrid tablet/laptop devices use heat-efficient CPUs and GPUs that are designed to eliminate the need for a system fan.
Overheating is The Enemy
Shutdown your laptop immediately if the fan stops working to protect it from damage. Producing heat is a normal part of laptop hardware use; producing too much heat is a very bad thing for a laptop. Laptops that overheat malfunction and crash in the short term; prolonged exposure to overheating wears down and eventually breaks hardware. It helps to know your hardware: if your laptop normally produces a heavy fan sound during activities such as watching a YouTube video or browsing the Web, and it stops making the noise, you may have a problem. Some fans may be inaudible in low-heat use, so try putting your hand near the vent to feel for airflow.
Check For a Fan
Inspect the laptop for cooling vents; this is useful for identifying if the laptop has a fan-based cooling system, and also identifies where to clean the system to remove obstructions. If your laptop produces enough heat to require a fan, that implies that without the fan, the laptop can produce enough heat to damage itself. Most laptops feature an exhaust vent on the side of the device and may include an air-intake vent on the bottom of the device. Some laptops like the MacBook Air conceal the vent in the monitor hinge.
Compressed Air is Your Friend
Cleaning a laptop's vents is a low-risk, non-invasive process that can remove any debris buildup that is preventing the fan from running. Laptop vents should be cleaned out every year or two. Clean the vent using controlled air bursts from a compressed air canister. Shut down the system, remove the battery, close the screen lid and flip the device upside down before cleaning. Attach the straw to the compressed air can and generously blast the air into each vent on the laptop. Avoid using a vacuum cleaner because it can build up static which can damage the device. Also, avoid blowing out dust with your mouth.
Professional Laptop Repair
If cleaning the vents doesn't fix the problem, something's gone wrong inside the laptop. The fan may need to be reconnected to the motherboard or completely replaced because the motor is broken. Unless you're comfortable assembling and disassembling a laptop, taking your device to a professional for repairs is the safest course of action. It's very easy to break a laptop by disassembling it incorrectly and can be very difficult to put it back together.