DIAGNOSIS The compressor isn't kicking on, so it's just running the fan. This could be a refrigerant issue (low charge), a faulty compressor start component, or it's too cold in the room.
FIX Check the room temperature first. These units have a cold-temperature sensor and won't run the compressor below about 41°F (5°C). If it's warmer than that, listen closely after turning it on. You should hear a distinct 'click' and a deeper hum about 3 minutes after startup—that's the compressor. If you don't hear that, the compressor circuit isn't activating.
IF THAT DIDN'T WORK If the room is warm and you hear no compressor, it could be a failed start capacitor or a sealed system leak. Both are serious repairs. A tech would need to check for power at the compressor terminals and refrigerant pressure.
HEADS UP If the coils are frozen over, it can also cause this symptom. Turn it off, let it thaw completely (might take a day), and try again. A dirty air filter can cause freezing, so clean that too.