Even Microsoft will do an unauthorized access investigation if your Xbox Live account is hacked, which can lead to bans being reversed. I sincerely hope that Valve's official position on such matters is something more constructive than "not our problem" -- particularly if the vulnerability used to perpetrate the hack was due to a failure on their part. (Which wouldn't be unheard of, as Steam itself was hacked in late 2011 and a bunch of customer data was stolen.) -- "The counsel of fools is all the more dangerous the more of them there are." -Ólafr Höskuldsson
usually Valve does this, but if they have reason to believe that it was a legit ban, they will leave it. My spidey sense says that OP cheated and was caught.