Has your computer accidentally blocked steam with a firewall?
Check to see if you have an internet connection... Post back if you don't, and I will e-mail you instructions how to get back on the net... Wait.. scratch that... you are online... (JK, I am not that dense. Was a joke, I swear! Seriously...

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In your control-panel, or on your network icon... Select the firewall tab and make sure STEAM is ALLOWED to use the net. (WIndows may have auto-blocked it, just now detecting it as a potential threat, accessing ports on your internet. Or someone may have selected, YES, Block this program from accessing the net. Or it is randomly blocking it... Or the servers in your location are down/busy.)