from the man page of git stash

Recovering stashes that were cleared/dropped erroneously
           If you mistakenly drop or clear stashes, they cannot be recovered through the normal safety mechanisms. However, you can try the
           following incantation to get a list of stashes that are still in your repository, but not reachable any more:

               git fsck --unreachable |
               grep commit | cut -d\  -f3 |
               xargs git log --merges --no-walk --grep=WIP