DIAGNOSIS The cooling system has failed. The most common cause is a refrigerant leak, but before assuming the worst, check that the compressor is actually trying to kick on. You'd hear a distinct click and a deeper hum a minute after turning on Cool mode.
FIX Listen closely. Set the unit to Cool, Temp as low as possible, Fan on High. After about 1-3 minutes, you should hear a click and the sound of the fan motor will change pitch as the compressor engages. If you hear nothing new, the compressor isn't starting. First, ensure the exhaust hose is connected perfectly and the unit isn't in an error state (like E2). The unit has safety sensors that prevent cooling if venting or water levels are wrong.
IF THAT DIDN'T WORK If all conditions seem right but no compressor click, the issue could be a failed compressor start capacitor or a control board fault. These are internal repairs. Given the cost and complexity, this often means the unit is toast if it's out of warranty.
HEADS UP Portable ACs are sealed systems. You can't just 'recharge' the refrigerant like a car AC. A leak means a major repair that usually isn't economical. This is the downside of the all-in-one portable design.