Deleted my whole piece

Have you looked in the hidden backup folder in your Scores folder? Google how to view hidden folders for your OS.

"I have had a piece I was working on for 2 weeks straight just to find the whole thing was deleted"

It is not entirely deleted, but this score is in a tangle. It seems to me that the "full score" is empty, and in addition the full score contains only two instruments (Piano and Violoncello).

But the separate parts for each instrument of the orchestra are all intact. So perhaps you can add back the missing instruments to the full score, and then copy-and paste each instrument's content from the part to the full score?

You should first ensure that your damaged full score has exactly the same number of measures as the parts, and the same key signature and time signature - and most important each change of key signature and time signature. Otherwise the copy-and-paste will fail.