AC turns on than clicks off

[Viva Las Vegas JDR]

hey guys I have been using the ac well early this morning I noticed it was hot and the ac would not turn.. so I shut down the front ac and started using the rear ac.. Later I tried using the front ac again but when I first turn it on you can hear the compressor click on and started to hum but after a 1/2 second the ac turns off.. the fan works when put into fan mode.. any ideas what I could test.. oh and the breaker stayed on the hole time..thanks in advance

JDR