DIAGNOSIS The overheating protection has kicked in. This is usually because the air intake or exhaust vents are blocked, causing the unit to overheat.
FIX Immediately unplug the fryer and let it cool. Check the back and bottom vents for any blockage—like a kitchen towel, crumbs, or it being pushed too close to the wall. Clear anything you find. Also, make sure you're not overfilling the basket; overcrowding restricts airflow inside.
IF THAT DIDN'T WORK There might be a significant buildup of grease and debris inside the unit, around the heating element, causing it to smoke and trip the safety. You'll need a deep clean (see the cleaning question) or professional service.
HEADS UP That burning smell is the safety system doing its job. If you keep running it with blocked vents, you risk a fire or permanently damaging the internal components.