Steam derps sometimes when it comes to load things from their server.

If there is something wrong from Valve's side, it is usually fixed within a hour.
Also: The offline mode of Steam got a few issues. Sometimes you can confirm that your user is legit (by logging in the offline mode while connected) at least once every two weeks; sometimes failing to confirm your user within that time may screw things up; sometimes the offline mode fail to work for reasons I do not know 'bout.
Valve has apparently tried to make it work as intended for everybody for a while, so we are left to see how that'll turn out in a distant future.
Also also: If you think the issue may lie with your connection, try to contact whoever is responsible for your connection.
You may also want to try a wired connection (if possible). It should be far more reliable for speed and stability (and security).