have you verified the SD card is good? A bad card / drive has a tendency to hang computers as they boot. See how far it will boot without the SD card in place.
This is from memory so might be way off base. Get a usb keyboard, plug into HMI, while booting, press 'ctrl', release 'ctrl', press 'alt', release 'alt', press 'del', and release 'del'.
I think this takes you to some menu, but its been over 5+ years since doing this.
Also, have you tried to remove power for at least a minute?