Problem with Chiller

[Jaws]

I have a Pacific Coast 1/4 hp chiller that has done a great job keeping the temperature between 77 and 80 for 4 or 5 months now. Tonight I noticed that it appears to be on all the time and the temperature won't drop below 81. It isn't rising above 81 but won't drop below it either. I checked the settings and even tried lowering the set temperature to 75 instead of 77. I tried unplugging it and plugging it back in but nothing there either. Any idea why this might be?