A couple of ideas. I agree, you have eliminated hard drive failure. Two possibilities exist: A failure of the main board, and a bad power supply. If the top of the large capacitor on the power supply is bulging upward, replace it.
A serial cable may give information on the failure; I seem to remember it defaults to the menu (or the password prompt) if no hard drive is found.
Since the system was unhacked, any chance of recovering recordings depends on getting the present main board working or swapping the crypto chip to a new mainboard.
There's a difference between needing help, and just being plain ole' lazy.
"You cannot teach a man anything. You can only help him find it for himself." Galileo Galilei (1564-1642)
HR20-700 with 2 TB, HR22-100, HR22-100, HR22-100, HR23-100 all running 0x5cd and networked.