From git stash --help
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
This helped me better than the accepted answer with the same scenario.