Strangely enough, try restarting your computer with the network connection active and relaunch Steam.
Or killall MainThrd and launch again.
I have the same problem. This happens also when I come back from suspension with no network. Close and reopen Steam doesn't work. The only way to get connection again is restarting Steam with "killall MainThrd"
I'm on Ubuntu 12.10.
So it seems that the core of the issue is that Steam doesn't close correctly (or rather, completely) for some users.
I can't reproduce this. If you are seeing this problem, instead of doing "killall MainThrd" do a "kill -11 $(pidof steam)"
This will cause steam to crashdump with a SIGSEGV and upload a minidump. Then restart Steam and post the CrashID (under Help->System Information at the bottom of all the text)
It should look something like this:
''/tmp/dumps/crash_20130114173503_1.dmp'', upload yes: ''CrashID=bp-fd69a34a-13ac-4098-b16c-6891b2130114''
This should at least give us some clues where Steam is hung.
Nothing extracted yet.
Started the steam client with the network cable disconnected and received the expected network missing dialog. Canceled from that, connected the network, but now the steam client will not start. Steam does not appear in the process list and attempts to execute from the command line appear to do nothing (the program reports no error, appears to do nothing and returns to the command prompt as if nothing happened - still no steam process is created).