DIAGNOSIS This is usually a thermal overload safety cut-off. The compressor or thermoelectric module is overheating because the vents are blocked or the room is too hot.
FIX Unplug it and let it cool down for an hour. Check the rear and bottom vents. Make sure there's at least 4-6 inches of clear space on all sides, especially the back. The room temp should be under 90°F. Plug it back in and try with cold water in the reservoir to reduce the initial cooling load.
IF THAT DIDN'T WORK The internal fan that cools the components might have failed or be jammed with dust. You might hear a faint hum or buzz before it shuts off. This requires opening the case to check, which isn't for everyone.
HEADS UP These are thermoelectric (Peltier) coolers, not compressor-based. They're inefficient and generate a lot of heat out the back. Putting it in an enclosed space or a hot kitchen corner is a sure way to trigger this shutdown.