If you ran a git pull and need to revert, you can use git reflog to find the last revision you were on before you pulled:

~/work/some-project (master) % git reflog
f7da659 HEAD@{0}: pull: Fast-forward
...
~/work/some-project (master) % git reflog
87582cf HEAD@{0}: pull: Fast-forward
f7da659 HEAD@{1}: pull: Fast-forward
...

HEAD@{0} is the commit you just pulled, and HEAD@{1} is the commit that was checked out before the pull. To revert to it:

git checkout HEAD@{1}

See man git-reflog for more info.