Update failed, restoring from app.bak?
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).