Adobe Prem Pro - Stills changing to green screen

I hope somebody can help. I am a new user for Adobe Prem Pro - I import a still jpeg to (say) bin01 and then drag it to the timeline, adjust settings ect. All seems to work fine that is I can then play the video and the still fades in an can be seen for the period defined (say) 2 seconds. Some time later, for reasons I do not understand, when playing the video (before export or after), the place of the still is now a green screen or black screen. The only solution to date has been to delete the scene and replace it. Any ideas?