From the http://www.uesp.net/wiki/Oblivion:Bothiel:
It is possible for Bothiel to disappear indefinitely into inaccessible areas. This seems to occur particularly if you are doing both of her related quests (Repairing the Orrery and Necromancer's Moon) at the same time. One known solution is to uninstall the Repairing the Orrery plugin; within 24 hours Bothiel should reappear in the Arch-Mage's Lobby. Complete Necromancer's Moon, then reinstall the plugin.
Normally I'd suggest trying a place at me console command, if nothing else, but since you're on the 360... :shrug: Temporarily uninstalling the Orrery DLC looks like your only option. Hope it works.