Steam won't let me play Half-Life2?

I have Half-Life2 so I load it into my computer and it said that it has already been registered and an email is being sent to the owner. Well I never got an email so the only thing I can think of is I must have had a different email address. I bought the game new many years ago. So is there anything I can do or should I just throw the game away? Also what games goes on Steam because some play from the DVD like "Enemy Territory Mine Quake Wars". Is this a crappie game and should just throw it away or am I missing something?

Buying Half Life 2 retail is basically buying a code because it has to be activated on steam. Is it not on your steam account? If not, did you register it on a different steam account? Whatever account you registered it to it's stuck to.