A denial is a refusal, and often means a refusal to believe or accept something as the truth. "When asked if he ate the powdered doughnut, he gave a flat denial. Since his face was covered with powdered sugar, the denial was ludicrous."
To be "in denial" is to be fooling yourself about the reality of your situation. If you stay up late watching TV on the eve of your AP Biology final, you are probably in denial about your lack of preparation for the test. Now, "self-denial is a whole other thing; that is when someone puts others first: "During the drought, Mary made sure all her neighbors had enough water and she lived in a state of self-denial — relying on Mountain Dew as her only source of hydration."