I have added green screen effects to my video preview is just how I want it but when i render it and once exported to my computer that when i got to play the video file that it can't play it? and the play lengh is 0:00 why is this happening?

I have try closing the program and starting it again that didn't work. Also restarting the computer still can't play it when i render it again.

Beforethis started happening i did do a previous video and worked perfectly it is just this video.

Please help a.s.a.p thanks!