99.9% of the time the person actually had cheats on their computer when they were banned. People sometimes cheat on non-secure servers and then fail to clean up their computer properly when going back on a secure server and such. It's come up so many times on the Steam forums and it's almost always a legitimate ban. They can even look up your account and tell exactly which cheat you were using. (There were some really early cases of people being banned when they had faulty RAM, but that problem was resolved.)
It's highly unlikely someone got your password and decided NOT to steal your account.
But anyway, you can submit a support ticket to Valve to have them look at it and maybe reset your account. Don't expect any success though. You can still play on non-secure servers, btw.