Whole Rest has dissappeared and won't come back.

I am attaching a picture of the file at the bottom.

I am currently writing a huge medley of songs from a Broadway show and I was nearly finished with the second song when i hit measure #43 and noticed that the top staff (RH) of piano where the whole rest is usually at when you begin to enter notes has mysteriously vanished and won't return.

So i tried to enter a rest. Nope, it didn't work, it won't let me enter anything.

So i thought maybe i deleted the rest by accident. Hit hit the back button and I didn't seem to delete the rest.

Finally i came up with the great idea of copying a pasting a blank measure (with rests) that is further down the page. Still nothing. Next i tried copying and pasting a measure that has notes imputed on both hands.
It just put them all down in the bottom bar of bass clef and the top one is still empty.

Has anyone else had this problem?? Please help!

Thanks in Advance

-Kaitlyn