Why doesn't my hot tub heat up?

It looks like it's running fine, it's on 120 volts and it's drawing 16 amps and the heat pump gets hot. It sat for a long time and when i tried to turn it on, it shut off and the keypad read "hl" and then it re-started itself and it shut down again and then the keypad read "flo". After that, I had to manually shut it off and turn it back on, i ran it all night and it seemed like it would work but the water just got colder.