"No Video" just as I'm about to start a mix

Hi Guys

My video (top left of Soundtrack 3 screen) has disappeared just as I was about to commence a mix.

Just above the tracks (where the video usully resides in blue) the video track is empty and I see the message "No Video" next to a small blue logo of a film strip.

Any tips whatsoever would be greatly appreciated as to how I can fix this.

Warm regards

Peter