Compressor will not kick in if refrigerant is low. While it could be a relay or fuse, this is the most common issue.
Did you ck the fuse? Note: it's never been the fuse for me...no such luck...
If refrigerant has left, and if leak can't be found, a good shop will add dye to the new refrigerant to trace the leak.
I did not realize the compressor would not even start if the pressure was low. But the fuse is also a good idea to check! We noticed the wiring the was chewed thru the tape around the radiator shroud but fortunately no wiring there was compromised.