power on the Ipod (even if it stays blank) and then immediately reset it. Also, another issue might be an empty battery -- connect it to a wall charger and wait at least a bunch of hours. The apple firmware that manages booting (which isn't replaced or modified by Rockbox) has known issues with sometimes reading wrong battery values (and not turning on / immediatly powering down because of this).
You can't brick an Ipod by installing Rockbox.