I have a spead sheet set up as a time card that is used several times a day. Sometimes when I go into to it the time card is gone and all I have is a gray screen like the sheet has been shrank down. Problem is the toolbar to make it full screen is also missing so all I get is a blank grey screen. To get this spread sheet back/viewable I have to go into a new excel session and make that sheet a full size. Once I make the this one full size I can get out and go back into my time card and it will be back full screen.

Any ideas on why this happens and if I can keep this from happening?
Thank you