uhh...in order of most likely fixes
1. Verify internet connection (open google
)
2. Close the game and open it again
3. Restart steam
4. Delete the clientregistry.blob file in the steam directory and restart steam
5. Restart your computer
6. Restart your router/ modem (or both)
7. Ask steam support or someone who has actually had this problem
Actually now that I think about it...these steps could work for virtually any problem with steam