air conditioner not working

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Air conditioner was working fine and then on way home it suddenly started blowing hot air. Once home I checked unit and it wasn't and wouldn't engage. was told by parts person there were 2 switches / one high side and one low side and that when unit was on they should have c urrent to them. Low side does but for the life of me I dont know where the high side switch is -- HELP. I was told that if either of these switches weren't working the compressor wouldn't engage. Is this correct?? Does anyone know what else might be the problem??