I would post this in Bug Reports but I'm not level 7 yet.
Bug was encountered on EUW server.
This was the first time this bug happened - I have played as Garen before on the EUW server and not had this happen.
Throughout all of the game, Garen was consistently moving towards the opposing spawn. I was unable to use my basic attack but could use abilities. Damage could only be dealt using the Judgement ability. Movement could only be stopped by recalling and cancelling.
I don't think this glitch can be reproduced. As mentioned above, I have played as Garen on EUW before and it's been fine. I have not tried to recreate this bug as I encountered it at about 22:30.
Log file: https://drive.google.com/file/d/0B0TmPI33X3WpRjJIeHEzeXVqWG8/view?usp=sharing