I came across this question before. Today, I encountered this scenario after my VM hung and I needed to reboot the server.
Very strange. Everything was running fine for so many days. I panicked when I have to re-configure again.
Well, after I cooled down, I then realized my OpenDJ was not started yet!! I had my OpenAM installed with configuration and user data stores in the external OpenDJ. (I did not use the embedded OpenDJ that was bundled with OpenAM)
Ok, so I shutdown OpenAM. Started OpenDJ, followed by OpenAM.
Everything is now back to normal. Phew!
Anyway, I would expect OpenAM to be smarter. At least, it should check whether or not there is an existing instance installed by looking into the .openssocfg directory.
If existing instance(s) found, then it should not redirect users to the Configuration Page.