Contact Riot Support via their page (Click the support buttons at the top or bottom of this page) if you're not sure why you received it.
Also, more info please.
If it was for Behavior, you should've received a reform card upon logging in/via email.
If it was for use of cheats/scripts/third party modification, it should say something to that effect.
If it just gives an absurdly far away year and nothing else, it probably means they locked it due to suspicious log-in activity or chargebacks on purchases. Contact Riot support to go about getting log-in access back (securing your account or paying back what is owed)