I had a similar problem with Team Fortress 2 and it was because the game had released a patch and it was in the process of downloading it.
Maybe Steam knows that tehre is an update but your settings are not set to download it. Try Right clicking the game in your list and going to Properties, then Update and check to see if it is set to Auto Update. If it is, change it. Close down Steam. Restart Steam. Then change it back to Auto. Then Close down Steam and restart it.
THen u could also goto the bandwith monitor to see if it is trying to access any connection.
Deleting the clientregistrybob might also push it along somehow.
Check and validate the games files incase they have become corrupt.
Other than that when it happend to me on TF2, I just waited for the game to fully download and then it loaded up. It was like it didn't realise that there was an update, then when i tried to launch it saw it and then it just freaked out. This might not be the case for you if it has been happening for a few days now.
As for the replies in this thread, just try and help rather than bitch. Even though you think it's a lack of information, I myself have had this problem and I can suggest some things to try even with the information given. No need to go all 'Police like' on the guy.