FW400 HDD not recognized at all
First, will the FireWire port work with another external hard drive with FireWire? This will rule out if the port has failed or not. IF you have another Mac nearby but not another external drive, another way to test this would be to see if the drive will mount on another Mac.

Second, can you dismantle the external enclosure and move the bare hard drive to another external enclosure, and see if that works? This will rule out whether the interface hardware in the external enclosure has failed.

If you don't have the equipment to test this, take the system to a repair facility that does have the equipment and can test it for you.

If you are fortunate, the bare hard drive is OK and only the enclosure will need to be replaced.