Why is my air conditioning not cooling?

The air conditioning stopped working all of a sudden. It was on cool and auto. It was set on 70 degrees. The temperature in the house is 80 degrees. I tried turning it down to 65 but it still will not cool. The air is coming out the vent but just not coming out as cool as it should. What can I do to fix this without calling a repairman and paying an arm and a leg?