In case you have reverted back to a past commit and lost a newer commit you can recover the lost commit by running
git reflog
Then find your lost commit, and reset back to it by doing
git reset HEAD --hard <sha1-of-commit>
In case you have accidentally commited a delete on a file and later realized that you need it back.
First find the commit id of the commit that deleted your file.
git log --diff-filter=D --summary
Will give you a sorted summary of commits which deleted files.
Then proceed to restore the file by
git checkout 81eeccf~1 <your-lost-file-name>
(Replace 81eeccf with your own commit id)
To restore a file to a previous version you can use reset.
git reset <sha1-of-commit> <file-name>
If you have already made local changes to the file (that you do not require!) you can also use the --hard option
To recover a deleted branch you need to find the commit which was the head of your deleted branch by running
git reflog