if it had a contact email change without u knowing, then yeah, very likely been hacked
did you have many games on it? cos with you saying you finally got a PC that can play games on it i'm thinking that perhaps you didn't have many and maybe it'll be cheap to replace your account.
that would be worse case though, and i bet a few here have spare copies of HL2 that came free with later version of HL2:Ep1 etc, so some stuff could be replaced, but hopefully you can get your stuff back.