Account compromised and banned

While at work I received this lovely email stating my account was banned. "Your account: ** was banned due to the use of malicious 3rd party programs or modifications to your League of Legends client."

I've been playing for years and I've never once used a third-party software. However, now that I got home and verified that it is indeed banned, I dug a little further into my account, specifically the match history though this site. I notice the last 6 games today are not ones I've played, this leads me to believe my account was compromised. I've never intentionally shared my account information and to be honest the idea of anything of mine being compromised is frightening. I have since changed my League account password (along with many others).

I can imagine whom ever compromised my account has free reign on whatever else they use, but is this actually punishable? It's not like your client REQUIRES the implementation of any real security measures more than just a password.

Is there a chance for my account to be reinstated? As far as my play history, the machine, and IP (Comcast) for the majority of my play history should vary only slightly.

Thanks.