How do I restart a minecraft server stopped with /stop?
Unfortanly, the only way to start the server again is to access the PC/server/box that was hosting it. That /stop command is only meant to be used to shutdown the server so the admin can make config changes.
Well, you can't.
But the owner can do something about it. He can end the process, and start the server, or restart the server. If he cannot do that, that's a crappy hosting.