Which version of PhotoStory are you using. I have 2013. It's hard to lose the main monitor as PhotoStory does not allow one to move the windows around.
Do you mean that the image no longer shows up in the monitor area? If so, click on the down arrow beside 100% at the bottom right of the monitor area and select 100%. You image should come back into the monitor.
If it is something else, go to File -> Setup and at the bottom of the list select Reset program settings to default.