If you do Ctrl A that marks everything as selected - then shift K and recalculate picture files.
 
Also just changing your base folder for projects in settings, starts a new picture file folder under that - used for all newly built picture files - this even if you open a project in your old project folder, Sonar looks if present in new Picture Files folder, if not there builds it.
 
This will not change in next Sonar version.

I tried that selecting all and recalculating picture files earlier by hitting shift + k but nothing happens. I guess it's because the project looks like I've just hit file --> new and only added plugins and buses. Clearly something went very wrong somewhere the last time everthing was ok and I saved. Thanks again for the suggestions.

Use the Project -> Audio Files dialog to find what the project contains.
 
Is that empty - no files listed?
Do file path to files look alright?
 
The Audio Finder tool might also help CWAF on Utilities menu.
Local help refers to use online help for how that works - don't know why.
 
Save as bundle files also include audio files - also recommended if changing the global audio folder to get audio files where they are supposed to. So unless using per project audio files, you audio will be in global audio folder.
 
Help recommend having changed global audio folder, which might happend between Sonar versions - save as bundle with one version of Sonar, open this bundle with new version of Sonar - and all files will be placed where they can be found.
 

The paths on Project -> Audio Files are correct but the file list is empty. That suggests that all the audio files I had added to the project were somehow removed. I could add all the files back manually (along with the MIDI notes) to try to recreate the project but this data wipe might happen again in the future. That's why I really can't use the new Sonar at the moment anyway.

Is it possible that Sonar Professional is in demo mode?

So your files are in global audio folder - that could mean you are not using the "per project audio folder" in preferences.
 
Since this is default on by installing new - if your 8.5 have this unchecked - that might be your story.
New Sonar has a bug relating to global folder only, or similar.
 
I think most of us prefer using per project folders - so this was never discovered.
Something with save and not per project folder bug, kind of.

It's not running in demo mode, at least not that I can tell. This is not just about the missing audio files. Gone are also all the automation lanes I had added and the MIDI notes that are drawn to a MIDI track in the Piano Roll view.

If everything you've added or changed is gone, that sure would sound like it's operating in demo mode.
 

If demo is the case, and it really sounds that way - a good idea might be to warn at exit "your work is now lost" or something. Nobody should have to experience what Sonadi did.
 
I think gokidsmusic and stevec nailed it...

Check the Sonar About window under the Help menu. It will tell you if you're in demo mode.