The most common cause is dust inside the projector that leads to overheating & auto shutdown. If you have an air compressor you can use the blow tip accessory to blow as much dust out as you can without taking apart the projector. You may get by using a can of compressed air available at Walmart, etc. electronics section. Concentrate on vented areas & where you can see or hear any fans. Do this outside so it doesn't make a mess. If you know how to take out the lamp there's usually close access to the main cooling fan through the lamp compartment, blow air through there & it will follow the right path to exit correctly without blowing dust all over the inside.

Clean the air filter