DIAGNOSIS: The oscillation mechanism is either jammed by something or the internal gear motor has encountered a problem. Debris, a cable, or just normal wear and tear can stop it cold.
FIX: First, unplug the fan. Look closely around the base where the fan column sits on the motor housing. Make sure no cord is wrapped around it and there's no visible lint or hair jamming the gears. Gently try to rotate the fan head by hand to see if it's physically stuck. If it moves but the motor won't turn it, there's often a manual reset. For many Dreo models, you press and hold the oscillation button on the remote for about 5-10 seconds while the fan is on (but not oscillating) to try and re-sync the motor.
IF THAT DIDN'T WORK: If the motor hums but doesn't turn, or the reset does nothing, the internal plastic gear that drives the rotation might be stripped. This is a common point of failure after a lot of use or if the fan was forced while jammed.
HEADS UP: Don't force the head to oscillate while it's powered on, you'll almost definitely damage the gear. If the gear is stripped, you're looking at a disassembly job or warranty claim, not a simple fix.