Undoing a commit in TortoiseSVN

The easiest way would be to find the head commit of the branch as it was immediately before the rebase started in the reflog...

git reflog

and to reset the current branch to it (with the usual caveats about being absolutely sure before reseting with the --hard option).

Suppose the old commit was HEAD@{2} in the ref log:

git reset --hard HEAD@{2}

In Windows, you may need to quote the reference:

git reset --hard "HEAD@{2}"

You can check the history of the candidate old head by just doing a git log HEAD@{2} (Windows: git log "HEAD@{2}").

If you've not disabled per branch reflogs you should be able to simply do git reflog branchname@{1} as a rebase detaches the branch head before reattaching to the final head. I would double check this, though as I haven't verified this recently.

Per default, all reflogs are activated for non-bare repositories:

[core]
    logAllRefUpdates = true

You can use

git checkout -- file

You can do it without the -- (as suggested by nimrodm), but if the filename looks like a branch or tag (or other revision identifier), it may get confused, so using -- is best.

You can also check out a particular version of a file:

git checkout v1.2.3 -- file         # tag v1.2.3
git checkout stable -- file         # stable branch
git checkout origin/master -- file  # upstream master
git checkout HEAD -- file           # the version from the most recent commit
git checkout HEAD^ -- file          # the version before the most recent commit