Did it ask you to Login again? If so, somebody else might be using your account, and when they logged in, you were kicked off.

Connection lost, is just how it sounds. Have you done anything to your computer prior the problem? And have you checked your firewall and open ports to check and see if traffic is getting through? Here are my suggestions.

1. Log out and log back into steam, that has fixed countless net-related steam errors for me
2. Make sure your firewall isnt blocking Steam.exe or any ports for that matter... (I know it really shouldnt be if it worked before; but you never know)
3. Open the ports found here if you haven't done so already (Theres also a connectivity test on that page; try it out)

If all fails scream at your Router/Modem, Good luck

Dont worry about what Sgt said, if someone logged into your steam account you would be asked to re-enter your password because "your steam ticket has expired"

EDIT:

There is also a connection limit for cheap routers like mine, they reject packets when the limit is reached and you have to wait for 2 min for it to free up

If none of the above suggestions work, this thread is good for troubleshooting Steam problems:

Official Steam Problem Help Thread.