You all know how I feel about Steam...
Anyway, after having this particular issue, I wanted to post a solution. This worked for me and I didn't loose any game settings.
Steam Error: "Game unavailable. Please try again at a different time."
Solution: Please exit Steam and go to the folder called C:\Program Files\Steam\
(this is the default location for a Steam installation - if you set a
different installation directory, you will need to browse to it). Delete
all of the folders and files in this folder except: The \Steamapps\ folder and
Steam.exe (this file is listed as an application and features the black
and white Steam logo) Restart your computer. Then, launch Steam.exe from
within the Steam installation folder, and not from a pre-existing
shortcut. This will reinstall everything again. Then, with Steam running, re-test to see if the original issue was cleared up. Note: This process will not affect your currently installed games.
If the issue persists, make sure your network is optimized for Steam:
Troubleshooting Network Connectivity Link: http://support.steampowered.com/kb_a...1456-EUDN-2493