Help with simple display problem

The top bar containing the time/date on the right of the screen and also the apple logo/safari/edit/view/history/etc buttons on the left side is not displayed, despite it having been so before. I have to move my cursor upwards off the top of the screen to access it, by bringing it down into view. i know there is an easy solution to fix this, but cannot for the life of me figure it out. Help, please!