Pause does work then? I guess I don't trust it.
I'd go with Knightemplar1960' 'steam issue' or 'windoz issue'..
if the windoz key works or you can start task manager you
could screw around setting either the game or steam to
'high priority' and see if that does anything but you'd
probably spend less time just shutting the computer down
and starting over.
If it's still frozen after maybe 3 hours... how about disconnecting your
internet and then reconnecting? Maybe that would nudge steam a little.