Scenario: you notice that a file is deleted, and you want to recover it.

Find which commit delete the file

git log -- docroot/user_bars/logo.png   # show all changes of a file, work even if the file is deleted  
git log -1 -- docroot/user_bars/logo.png # limit the output of Git log to the last commit, i.e. the commit which delete the file; -1 to see only the last commit, use -2 to see the last 2 commits etc...

Restore the file

git checkout f5ac172e docroot/user_bars/logo.png # warn: this will discard all changes in working directory

If there are many revisions, you can view content of the file in a specific revision by the following command:

git show f5ac172e docroot/user_bars/logo.png