You can restore your database from your pre-update attempt database backup that you (hopefully) made. Assuming you've got the the backupDbOnUpdate config setting set to
true, Craft made one for you in
craft/storage/runtime/backups as well.
You can also download any previous version of Craft's files via: Where can I download previous versions of Craft?
Just replace the existing
craft/app folder with the fresh one you downloaded and delete
app.bak. I wouldn't recommend restoring directly from
app.bak because depending on where your update failed, it could be in an inconsistent state.
I'd recommend doing a manual update when you're ready to update again as you probably ran into your server's max execution time or memory limit (which tends to happen on these larger updates).