After a bug where Snap randomly crashed Google after extreme lag (unknown cause), all costumes that were hidden were deleted.
What do you mean by that?
Are you sure it was the crash that caused the costumes to be deleted, or had the project not been saved after they were added?
Please read Section II.E, page 28, in the Reference Manual for advice on recovering them.
Basically, my friend made a .PNG in MS Paint and I imported it. After that, Google kept 'fake crashing' (the screen goes black and then all the pages open bug out). After a while of this chaos, I later opened my project on a different computer and every costume that was not on the screen (all except one) was turned into a blank costume.
I was able to recover what I lost. Thanks!