So, you've replaced the battery. I know this is an obvious one, forgive me, I am not trying to insult your intelligence, but did you put the battery in the right way? Even if the battery is dead, it should still detect your hard drives and cd roms. It just won't save any custom BIOS settings, they will always default. Sounds like the motherboard did infact die, I am sorry to inform.
I'd replug them, check the cables and such and go into the bios settings. Try to change something and save it (the time for instance). If it saves, yahoo! lol if not, I think either the battery replacement is dead too or you are SOL my friend.
Was the fan noisy before this happened? It might have overheated and killed something (processor, etc).