I've had this happen and I know there's a FAQ on the problem on their support pages but I'll give you the cliff notes:
1. Restart steam
2. If that doesn't work, delete clientregistry.blob in Steam's main folder and restart (should force an update)
3. If that doesn't work delete everything in Steam's main folder EXCEPT steam.exe and your steamapps folder. This will make Steam do a fresh install.
When I had this happen to me, it was step three that worked for me.