Is the footage visible in your timeline UNTIL you try to play it back? If not, when you double-click on the footage to open it in the Layer panel, can you see the footage then? What are the specs of your clip?
Did you drag the clip into an existing comp or make a new comp based on the footage dimensions by right-clicking the clip in the Project panel and choosing "New Comp from Selection" or dragging it to the New Comp icon?
Also, how are you previewing, are you hitting the spacebar?
Lots of questions here, I know, but there's not a lot of info to go on. Check out this resource to learn more about how to preview in After Effects. Preview without rendering in After Effects