If you are in a public server, such as the NTH 1.2.1 server, and it crashes and you get that "warning_connection" problem on your screen press the ` button next to the 1 key, and type in "retry" and it will try to connect to that server agian. This works on public servers because after they crash they just start over. But if it's someone;s own server and it crashes (someone with 0 ping) then you have to leave and go find a new server.
That's the only restart thing i could think of