Unless you have a backup somewhere else, this is difficult. However stashes are stored as regular git objects and provided this was done recently then the commit object will still be there and not garbage collected yet, it just won't have any references to it.
You can list all unreachable commits , but you have to drop to the command line (click Terminal on the toolbar). Then type:
git fsck --unreachable
This will show you all the commits which are not reachable by a branch / tag and which haven't been garbage collected yet. Look for the ones that say 'unreachable commit', ignore the blobs. The chances are it will be the one closest the top, unless you've performed some other actions which created more unreachables. You can examine the commits by doing 'git show <sha>', and this should identify the stash, it'll probably be called 'On master: <your comment>' or similar. Once you've found it, copy the SHA.
To recover this you then do:
git stash apply <sha>
Which will bring it back into your working copy.
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