Trane compressor not starting

Two days ago I turned my thermostats to the "Off" position as I was not planning on being in the home for a couple of days. After returning, I switched both thermostats (upstairs and downstairs) to "Cool" and lowered the temp to 76 degrees. I soon realized that the air blowing out of the vents was not cold, but just the circulation from the fan. I found the problem to be the outside compressor, it will not start. It makes a humming noise, then a few clicks, then stops making noise altogether. I've tried resetting the circuit breaker with no luck. This unit was working perfectly fine two days ago, so I'm really stumped. The fan blows air throughout the house whether the switch is on "auto" or "on". The model in question is a Trane Heat Pump, model XE1000 High Efficiency Weathertron, it is 4 years old. Any help would be greatly appreciated as I'm a college student on a tight budget, thanks in advance.