A/C not working

I have had this car for two years, and up until a month ago the A/C worked great. Coming home from work one day it quit cooling. Two cans of refrigerant were added, it still did not work. I took it to a mechanic, he added more refrigerant to the car, it worked for one day then stopped again. The mechanic had added dye, and said there was not a leak, so he changed the computer that controls the A/C, it worked for one day again, and now it still does not work. the mechanic said he cannot figure it out, and now I am out $800.00 and still no A/C. Any help would be greatly appreciated.