What just happened?

[Sed Bratton]

I literally just beat the game about one minute ago and I have no idea what just happened. Now, I couldn't finish playing Half Life over Steam because it was bugging out, and I don't know the storyline for that, so could someone inform me on what the hell just happened?

 

[JackFrost]

You completed your mission to the G-Man's satisfaction; he therefore stopped time and pulled you out, presumably taking you back to whatever form of stasis you were in between the end of HL1 and the beginning of HL 2. And giving you his boogaloo jive in the process.
As to what happens to Alyx and everybody else...speculations ahoy.