DIAGNOSIS The kickstand hinge has likely worn out or the internal friction mechanism has failed. This is a reported point of failure on this ultra-slim design.
FIX There is no user-serviceable fix for the internal hinge. Your only immediate solution is to use the monitor in a different orientation. Lay it flat and use a separate tablet stand or monitor riser behind it for support. Alternatively, use the monitor in portrait mode if your setup allows, as the weight distribution may be more stable.
IF THAT DIDN'T WORK If the kickstand is physically detached, check the four small Phillips-head screws on the back cover near the hinge. They can work loose over time. Tightening them carefully with a precision screwdriver might restore some tension, but do not over-tighten.
HEADS UP The kickstand is not designed for forceful adjustment or for supporting the monitor on soft surfaces like a bed or couch where it can sink and over-extend the angle. Always adjust it on a hard, flat surface.