Do 'poweron' at the sc> prompt, if you haven't already.
Then do 'console' to get onto the machine console.
If the host is booted properly, do 'shutdown -i0 -g0 -y' to get to the OK prompt.
If the host is hung, or otherwise borked, send a break (Stop-A if you're on a local keyboard, Ctrl-] then 'send break' if you're telnetted onto a terminal concentrator. That should bring you down to the OK prompt.
Then just do 'boot -s' from the OK prompt, and you should boot into single-user mode.