I was in the prerig session for autorigs and then saved the file. I later reopened it but do not see the autorig interface where I can click on generate. it is all greyed out an only says create new autorig. How do I load that panel again without creating a new autorig panel?

hi Master Class recording in progress

to answer you question
in the Autorig panel:
File > Load Existing Asset, then select the HDA in the chooser that pops up.

note - currently Houdini and the Python Panel don't talk to each other about everything, so when you're working on a rig and need to close you file etc, be sure to save the rig by using the File > Save option in the autorig panel, this will save the current settings to the HDA for re-use.