Question about air conditioning system...please help!?

Your problem is that your contactor inside of the airconditioner arced and welded together... I have seen this many times... When this happens your airconditioner will run even though you have the thermostat shut off... You need to have your contactor changed... Not sure if you can preform this task yourself... You might want to call a service tech for this one... A contactor usually costs about $35, Labor charges vary between companies...