DIAGNOSIS The fan isn’t properly seated into its base, or the locking mechanism in the base isn’t engaged. It’s a simple assembly issue.
FIX Let’s reseat it. 1) Unplug the fan. 2) Carefully lift the entire fan body straight up and out of the circular base. 3) Look at the bottom of the fan column and the inside of the base. You’ll see a groove and a corresponding tab. 4) Align the tab on the fan with the wider opening in the base’s groove. 5) Push the fan body firmly down into the base, then give it a clockwise twist until you feel it click and lock into place. It should now be secure.
IF THAT DIDN'T WORK Check that the plastic locking tab inside the base isn’t broken. If it is, the base itself will need to be replaced. You can often find just the base assembly sold separately.
HEADS UP Always carry the fan by its base, not by the tower. Lifting it by the tower can cause it to separate from the base and fall, which is likely how it got misaligned in the first place.