Many things can make an edited polygon disappear, such as layer definition or page definition queries, categorized symbology with no default checked, editing a selection created layer, etc. If you can move your layer into a new project and things work, then either edit from the new project or look for something like the foregoing in the old. Editing in a new project also gets around a corrupted original project. I suppose another possibility is bad geometry. Or if you don't wait long enough an incredibly slow redraw, might tie up your display or your perception of it.
