I am so sorry to tell you that there isn't a way to recover the work if the app closed/crashed. In some instances, it will bring the file back but if it was not done the after the 1st launch, it is gone. I would start 'saving' a copy of the file in the future.
The basic workflow is:
a. Start a sketch.
b. Start a new sketch and save the sketch that you are working on.
c. Go back to the original sketch (in step a) via the Gallery.
d. Work on sketch (in step a) and then start a new sketch. At this point, you can 'Save a Copy" of your work.
David Lau