first go to different dealer if u got hosed by that one.
second, only few possible issues that could arrise from electrical or mechanical. ok mechanical could be compressor dead but ya probably know that most times i have seen is loose of refrigerant and the low pressure control does not kick in the clutch to engage the ac. most vehicles in older one, yours might be have newer controls would leak on the seal in front of the compressor to the clutch. WHY because winter the seal would shrink and let out the gas. now yours will turn on the ac in winter even in heating defrost mode. other senerio could be a leak someplace. electrical could be a fuse or line cut or even dash controls.
or as simple as user issue
. good luck. you should be able to see the ac clutch engage and disengage in the engine bay. need to have proper gauges to see what ya pressure is in system. but be carefull high pressure can burn ya if you touch it.