Check your keyboard connections, both at the motherboard and at the keyboard. If the keyboard is connected, you should hear a beep (from the keyboard) at power-on.
It is possible that's what's causing it not to boot is that it's in diag mode. Without console output from the PROM, you're not going to be able to determine that or change it. If your keyboard connections can't be fixed, and you really can't find another type-4 or type-5 keyboard, then you may want to unplug that entirely and go with the serial port. If you can get the PROM prompt, and if my guess is correct that it's in diag mode, then the commands you want are setenv diag-mode? false reset Good luck! p.s. I don't want to go on the cart!
