How to recover LOST WORK resulting from the use of git?

How to recover LOST WORK resulting from the use of git?

I'm using SourceTree and what happened is that I clicked on "Commit", then
added all changed files, but before committing I wanted to remove one of
the files in order to commit it later.
I right-clicked on my file and was offering the following choices:
"Discard",
"Remove",
"Unstage from index",
"Stop tracking"

I was really confused by those choices, I clicked "Discard" and now all
the work done in this file was lost!!!
PANIC MODE! How can I recover my changes in this file?
Thank you!