My steam got hijacked a few weeks back as well. They used hacks on CS:S and got me a stage 2 VAC ban, which includes HL2:DM, CS:S and DOD:S, all of which I had.
I got my account back, and at least I can still play L4D and L4D2. Even though I can prove it wasn't me hacking, they will not disable the VAC ban.
I'm pretty sure it was someone from these forums who hacked it, because the hacker was from Everett, Washington, and the only Americans I know are from here.