BLACK SCREEN When I output render

Soemone please help me. Im kinda new to blenda and I try to save my file to AVi but when I play it the screen is all black. I checked the frames e.g 125 and then rendered a preview shot and it looks fine, so whats happening?

did you actually render the frame this time?

see, because playing back an anim will load frames of the same names, if you had rendered another animation before without changing the output path or name then when you play it now, it may play the frames of the other animation