You have likely lost some object off to never never land beyond the perimeter of your drawing space.  If you get the same behavior from opening a perspective overview camera then you need to go to plan view close other camera views and look for some object around the perimeter which can sometimes be quite challenging.   One tool that I find useful is the "Fill Window" tool [F6] or select all [CTRL +A] that will include everything in the plan including your mystery object and likely will make you