Logged in from a different computer, can't play on servers anymore?

So I logged into Steam on another computer, and now, whenever I try to enter a server in Gmod, I get this message:

http://img179.imageshack.us/img179/6726/dafs.jpg

I press the “Login” button, but nothing happens. What am I supposed to do? No one is logging in from another computer anymore, it’s just on mine.

http://www.facepunch.com/showpost.php?p=17773233&postcount=2

Search next time.