In the SMS menu, when you scan for boot devices, it should finally show you your disks and the AIX release (not TL or SP, just release) that is contained.
Is this your private machine? You might be able to reinstall it from NIM (on Linux -
How to install AIX 5L).
In a corporate environment you might already have a NIM server, otherwise configure one.
In both cases define the resources based on the installation media you have. Boot from NIM into maintenance mode to change the password or reinstall the system.