You core dumped for some reason. It could be because things couldn't be shut down before you booted. I would try the whole routine again: boot (or press ctrl b), interrupt boot, bo pri isl, hpux -is, passwd root.
If that doesn't work, another reason it didn't boot might be that the disk that is listed in the system as the primary boot disk isn't any good any more for some reason. hopefully, this system's vg0 volume group was mirrored.
If trying the simple way doesn't work again, before you type bo pri isl, there is an option you can use to search for bootable media. I think it's sea.
select that and you may have to go through several attempts to get to the media that will boot.
Does this system have a recent ignite tape that you can use to boot from? You also may be able to boot from the installation CD. I believe there is an option that allows you to just change the root password.