No folders showing in Navigation Pane

If you can still see Collections and images inside those, this is not an empty Catalog accidentally opened.

The Folders panel is in effect a dynamic "database report" of what location information is held against every 'imported' file - which merely means, 'registered' - and summarising all of that in the form of a clickable listing.

This listing is I think not continually re-generated afresh, but cached - and updated to follow your actions such as removing or adding images or their folders, or actively updating their location info. If that cached listing has become corrupted in this current Catalog then IMO that may explain the symptoms fully. Proper information is clearly there somehow on each photo, if that listing does successfully rebuild or partly so - within a given session of using the program. But that rebuilt listing is not then 'sticking' for a subsequent session.

I am presuming your storage location is not changing in address - for example, appearing under different assigned drive letters at different moments. If that were the case, this drive letter (or network URL) should first be fixed as constant and persisting.

One tactic for dealing with Catalog corruption that is safe to try: create an entirely new fresh Catalog, empty. Then use "import from another Catalog", selecting your current Catalog as the source for this - in this case including all contents (for other purposes it is possible to be more selective).

This function creates catalog records inside the destination Catalog, each corresponding to a photo present in the source Catalog. So that happens afresh, in a properly indexed and clean way, leaving behind any database structure problems that the old Catalog may have suffered from. If any particular image record was itself not properly readable I believe that image would be simply skipped over, so you would  be left with only all good data.

These records replicate not only all the latest editing but also prior edit history and all the virtual organisation: keywords, stacking, virtual copies, collection membership etc. Matching static Collections and Collection Sets, and Smart Collections, are therefore created.

If importing library into this new Catalog has successfully fixed the issue you can just go forward with that (and discard the old Caatalog). If unsuccessful you can abandon the trial (and discard the new one). There will have been no duplication of the image files themselves and no changes made to the old Catalog that you've merged images in from.

You can expect this fresh Catalog to ask for new Library previews to be generated. But you do not need to wait until that is completed, before you can establish whether everything you want is present. 

If this approach has not been successful: corruption in the source Catalog may have extended more deeply and you may have no choice but to fall back on restoring a whole Catalog backup: the most recent available which does not exhibit the bad behaviour.

You have accidentally opened a blank catalog. You need to go to File->Open Recent and try opening each catalog in that list until you find the one that shows your photos. If that doesn't work, use your operating system's search feature to find all file(s) on all disk(s) that have a name that ends with .LRCAT and then open each one found until you find the catalog file you want. And you do have recent backups of your  catalog file, don't you?