What do you mean by "afk ban?" Typically, if a player does not have any history of leaving or afking from games, they will not be punished for one or two sporadic instances of leaving. The system is set up so that players are not punished for things outside of their control, such as the power going out once over many months. However, if a player begins to show a pattern of leaving, like disconnects on consecutive days, it is possible that they could be punished by the LeaverBuster system.
This system begins with a warning pop-up, then will put players into a Low Priority Queue for a set number of games, where the wait time increases each time the player is punished (5, 10 and 20 minutes prior to joining the normal queue). The LPQ works quite a bit like the Instant Feedback System, in that there are punishment tiers, and each successive punishment will move you up to the next tier. Where the IFS will have you serve a chat restriction or a temporary ban, the LPQ gives you five matches to play. After you've served your punishment, you have a period of time where you can work your account back into "good standing", basically by not doing whatever got it into negative standing. If it was behavioral infractions, then you work on being positive, or at least neutral in matches. If it was for leaving/afk, then you work on not leaving or afking. By playing enough matches without showing any continued negative actions (be it behavior or leaving/afk), your account will begin to decay back into a positive state. It's a slow process, and it takes longer to get back into the good graces of Riot than it did to get out of them, but if you are dedicated, and patient, it will happen. As far as leaving, that means you just have to stop leaving. If you think the weather may get bad, and you don't know that you will be able to play, don't start a match. If you only have time for a 25 minute match, because you have to be at work in 30 minutes, don't start a match. Everyone has that feeling, that gut feeling that says "I wanna play, but maybe I shouldn't....." Listening to that voice, will help save you the hassle of a LPQ.
To the best of my knowledge, a player is only banned after going through the 20 minute LPQ multiple times. I cannot actually recall a case of a player posting here on the boards about being banned for afking without having gone through the 20 minute LPQ at least 3 times, but it's been a very long time since I've read about that sort of case. Can you clarify a bit more about what happened to your account, and maybe we can give you a bit more insight as to why it happened?
