Hunt... to answer your question:
SPOILERS!!!!
THe guy with the briefcase is G-Man, he came out in the first Half Life, and you saw him throughout the whole game, but could never catch up with him. At the end, he gives Gordon Freeman two options, one, die, and two, work for him.
So in HL2, it seems to be that you decided to work for him, and he put you in this "sleep" until the happenings of HL2, which take place a couple of years after HL1.
I was pissed off cause we still dont know anything else about G-man, or pretty much the whole story.

End Spoilers