2004 Mitsubishi Lancer

Recently my a/c stopped blowing air. My friend said I might need coolant, but then I told him that it doesn't blow anything out at all, not even hot air. Then a few days later, it started working again, and everything seemed fine. The air was nice and cool, and I just blew off the problem. Then the same thing happened again. It was working, I got out and went to the store, came back in the car, turned it on, and it didn't work. I drove a whole day without air, and in the morning when I got in the car, it started working again. What could be the problem?