OK, first make sure the books are really there. Right click on a book entry, and click "open containing folder". Make sure the book file is there and isn't corrupted.
If it's OK, you should be able to read any non-DRM'd file of most formats by right click > View > "View with calibre e-book viewer".
You can also try any other ebook reader by clicking the "Configure Calibre" icon to open the Preferences dialog. Under "Behavior" find "Use internal viewer for" and uncheck the file format. Then when you right-click > View > View, Calibre will use your OS default viewer for that file format.
If you have DRM'd files (such as from Amazon), you will need a plugin to remove the DRM. The best way to find these is to look on https://www.mobileread.com which is a good place for Calibre discussion in general. I used to know how to do the Amazon ones but it's been a while, there might be a new way by now.
What are you trying? Do you want to read things with Calibre's built in reader, another desktop reader, sync them to a device (i.e. use the desktop GUI to copy files to a device), download them to a device via the content server or what?
Which is why some people have taken to using the term crumble rather than collapse, crumbling happens at different rates in different places and can sometimes, at least temporarily, be fixed.
Once I put a book into Calibre, and make sure the metadata is correct, I delete the original. If there's any important info in the original filename I put it in the metadata. Is there a reason I need to save the originals?
Here's a simple one [1] I had some fun with. (There's a Flash version too [2]).
That one's just a game, so I have no idea how accurate it is. But if you search you might find something that's used in some university's nuclear engineering program or something like that.
Very realistic ones used for actual training exist - this one [3] uses VR! - but those probably won't be online.
I think many people don't realize that its appearance is very customizable - you can turn off all the icons, buttons, extra views etc., after which it has a very plain and simple appearance.
I get that tiny icons are in these days, but my icons are less than 40x40 pixels. Is that considered large? They look the same size as the icons in Firefox.
If it's OK, you should be able to read any non-DRM'd file of most formats by right click > View > "View with calibre e-book viewer".
You can also try any other ebook reader by clicking the "Configure Calibre" icon to open the Preferences dialog. Under "Behavior" find "Use internal viewer for" and uncheck the file format. Then when you right-click > View > View, Calibre will use your OS default viewer for that file format.
If you have DRM'd files (such as from Amazon), you will need a plugin to remove the DRM. The best way to find these is to look on https://www.mobileread.com which is a good place for Calibre discussion in general. I used to know how to do the Amazon ones but it's been a while, there might be a new way by now.