DIAGNOSIS: This is almost always a safety interlock failure. The fryer's base unit isn't detecting the basket is correctly seated.
FIX: Unplug the unit. Pull the basket completely out. Look inside the cavity at the bottom. There's a small, round, silver pressure switch. Press it firmly with your finger a few times. It should click. Wipe the contact area on the bottom of the basket tray. Reinsert the basket, pushing down until you hear a distinct, solid click. Plug in and try power.
IF THAT DIDN'T WORK: The internal thermal fuse may have blown. This requires opening the base, which is not user-serviceable. Contact Chefman customer support for warranty or repair options.
HEADS UP: Never run the air fryer without the basket fully locked. It will not power on, and forcing the switch can break the plastic actuator.