We need more info to work out what you are doing?
What are you caling the 'preview window" for instance?
I am working on a project for school. I have been draging clips down to the timeline and matching it with a few voice overs. Nothing too complex. I have been previewing my work as I go by pressing the space bar, Im not sure what I did but now Premiere will not play the preview in the Program sequence window.
Preview = The Program Monitor.
Are your source clips on an external drive? What are they from?
Is this laptop?
I am on an imac, all of the files are on my computer.