Selected Answer
Obviously, the blue window doesn't hold the missing workbook. Therefore the missing workbook is in a location other than where you expect it to be. Click the "View" tab on the ribbon and "Switch Windows". You will see all open documents listed. I hope that is where you will find your missing workbook.
Incidentally, while you are looking for it, there is a button to the left of "Switch Windows" with the caption "View side by side". That is the one you might have used to achieve what you wanted to do.
If the workbook is not listed in the "Switch Windows" dropdown not everything is lost yet. There is a chance that you opened the second sheet in another instance of Excel. Quit the instance which doesn't have the missing workbook. If there is another isntance running simultaneously it will come to the top of the heap and, with luck, your workbook will turn up there.