There are several reasons for this. One is that the low pressure switch is being activated by low freon. Another possibility is that a electricity surge went throught the unit and burnt a compressor wire. Still another possibility is that the compressor just locked up.
There are several reasons why: loss of power to the compressor (high or low voltage), damage to the control board, high-limit pressure switch in the open postition, or complete loss of refrigerant.
It could be many things first thing is to check the fuses on the electrical panel by the condensing unit. It could also be the compressor it's self take a ohm meter and put one lead on the case and the other on the power supply resistance is bad.
