you know, this actually happened on my own steam account that i am currently using...
goes back when the only color theme for steam was army green....
i was playing in a pub server and guy told me that he can give me admin ( i was like wut...12?)
so, being the immature kid that would give his liver for hot-wheelz, i gave him my pw and username.
after that my account was unaccessible and later on i gave up.
However, i went back to Valve and gave them the support questions and all that.
the guy who took my account did not know that i used CD-Keys for every product that i recieved.
HL-2, CS 1.6, Half-life Bronze pack.
Roxyness and Xenophek, thanks so much for sharing this to everyone and i wished that i could have read this 5 years ago.
Take another word from a 3rd "victum".
Ask an eGO member, and i'm sure that person will help you the best they can (including me!)