2003 Nissan Sentra Loss of Coolant

I have a 2003 Nissan Sentra, a month and a half ago my car was showing smoke out of my front defroster, I put stop leak in it, filled the radiator-it was fine. Two weeks ago, all of a sudden my car started running hot, I had no coolant in it at all, and my check engine light came on. The code said that the engine had run hot. I filled the radiator again, replaced the radiator cap and the light went off. Now, I had the same problem, I had no coolant in the reservoir and my check engine light came on again. I filled the radiator and everything, but I cannot find a leak. There is no liquid under my car, or under the hood, my floorboard is completely dry. Can any one help me?