Depends on the nature of the interruption.
If you walked away from the book mid way through you may as well PhotoRead it from the beginning. If it's a large book that you decided to PhotoRead in stages restart that section. Take a moment to close your eyes and reaffirm your purpose.
If you were interrupted to sneeze or the eyes glanced at something the kids or dog were doing while you were PhotoReading turn back a few pages and continue from there.
Once you are a confident PhotoReader you'll just be able to leave the book open at where you stopped. When you come back to it you'll turn back a few pages enter photofocus and start PhotoReading from there.
Always turn back a few pages, even if you go through the steps of entering Accelerated Learning State.
Alex