Got disconnected from a L4D game, restarted Steam, and it won't let me log in. Hmmph. Guess I'll try again later, but this has NEVER happened to me.

You tryed resetting your computer? Maybe it will allow you to log on after a quick reset.

Steam not letting you login isn't hat unusual. Restart Steam. If it doesn't work, delete the clientregistry.blob or restart pc. If it doesn't work, do what you didn't do before. It if doesn't work..try again later...

I had some real weird issue after the last update. The one that FINALLY allows you to select your download region (and sometimes specific site) as the primary source of downloads.
All games became set at "Downloading Paused" and would not run. Nothing appeared to fix it. Until...
I went and deleted a critical file from each game on the HDD, followed by the Verify game integrity whatsit and bang, 99% tick tick. game available and playable.
This means I've steam go nuts, about twice in the last 3 or 4 years or so. It's been good for me aside from the region locking and price gouging.