Ok so first you say you upgraded to Nuke6.5. Now you are saying that you don't have Nuke6.5.
I'm lost.
If you were running Nuke6.0 to begin with. And you were running phpBB Forums SEPARATE, then I don't know what to do.

If you ran any upgrade script, then the reason your Nuke 6.0 is not working now is because your Database was changed by the upgrade script.
That is why I asked if you made a backup of your DB first.

mikem