iMac is logging me out

My iMac is logging me out, and returning to the log in screen. AND when it does this, the log in screen is hidden behind a blue screen which i can erase by wiggling the mouse, (it erases in little square chunks) until i can see my log in icon (which of course I click to enter my password). then everything is fine until it goes to sleep and then it logs me out again. what is up with this and how do i fix it?