The discard (that you can see in "Undo Button in GitHub for Windows") is available to reset files added to the index.
That means you added files, and reset them before any commit.
The GitHub for Windows/Mac won't help you there.
You need to recover those files from the git repo (git fsck --unreachable --no-reflogs --no-cache HEAD
).
See "Recovering added file after doing git reset --hard HEAD^
" for more.
The OP Benz_java mentions "restoring lost commits", for looking in the reflog for lost commit.