OK, this could be about a million things.
Let's narrow it down.
Clean install of OS9? Install over 8.6?
Did you check to see if the driver is physically in the Extensions Folder (inside the System Folder). The file name would probably be DeskJet 800 Series.
If the driver is not there, reinstall it from the original CD, or the most recent downloaded drivers from the HP website.
Let's start with those and go from there.