Yea same here, It went down while I was doing something and I just came back when I came back.
 
Try CTRL+ALT+DEL for Task Manager:
  • Go to "Processes" tab
  • Find "Steam.exe"
  • Click "End Process" or right click and "End Process Tree"

If that doesn't work try restarting.
I tried all that but still it wouldn't work, when I enter my password and hit login, it would just freeze. (it was working fine couple of hours ago)
 
That too didn't work, Although I got it fixed by deleting the clientregistry.blob file and re-downloading it again. :D

Oh yeah I forgot about that fix... haven't had to do it in a while usually the process thing works. good job ;)