I've been using the website religiously for some time. It works great. Of course, to use it, you need an internet connection. Knowing I might want it offline I downloaded th app and the content I wanted available (the core books) months ago.

This week like millions (literally) of Californians I was without power and internet this last week. (All restored Yay!)  I was trying to do some game prep in my downtime (I had some charge on my device still). 

For whatever reason (update? passwords?) it kept failing to open any of my downloaded content.

This defeats the purpose of having it available offline.  Can you explain what I should have done, or what you can do to fix that?

Without knowing why it was failing, there's not much anyone can say. Content updates don't prevent content from opening and if your password was incorrect, that's not a bug or error.

If your devices local storage had become corrupted, then you'd need to refresh your files. Or maybe your authentication token was expired.

Are you by any chance in a campaign with content sharing turned on?  I ask, because a while back I tried to use the content in my app while offline, and I got a message about not being able to confirm shared content, and I couldn't access my content while offline.   Maybe that was it?  Or maybe it was trying to check for updates?  I agree it is frustrating when it happens.  (If it is a content sharing issue, they should just turn off access to that content instead of all your content, I would think)

Now connected to the Internet I am able to see the Dungeon Master's Guide; the Player's Handbook and Monster Manual just open but nothing displays. 

Update: I deleted and reloaded the App. It seems to work now. Not something I could have done without internet thought.