The first thing to check would be the collapsed view in the Folders panel.
If needed, right-click on a folder and select "show parent". Repeat as needed.
In all of them there are folders missing in my library. They are still on my hard drive but they are not in Lightroom anymore.
Is it safe to assume that these folders have photos in them?
Will I loose all my rating, editing and so on when I put via import (add folder)?
Don't understand this question. What folders you have in your library has no bearing on importing.
I have multiple catalogs.
Why? In general it is a bad idea to have multiple catalogs. Although I don't believe this is the root of your issue.
thanks for the replys. Half of the Folders are still in Lightroom library. On the hard drive there are all photos and folders. Nothing is lost from the photos. Some filders just do not appear in the library anymore. I also do not want to loose all my ratings, tagging and so on. I post an example below. Years 2009-till 2012 where there. Folder 2013,2014 and 2015 where missing. I added some files from the vacation I have been on but only the new one appear in 2015.
I have multiple catalogs.
-> Why? In general it is a bad idea to have multiple catalogs. Although I don't believe this is the root of your issue.
I do have multiple catalog because back in 2007 Lightroom was very slow for handling many photos. So I made catalogs for travel, portrait, landscape and so on. I do have 5 catalogs. If you know how to combine all of them into one, I would be thankful as well. The speed of Lightroom for MAC is good enough to handle all of them together I guess.
To combine multiple catalogs pick one to be the master or start a new catalog (File>New Catalog) then import the others into it (File>Import from Another Catalog...) DO NOT re-import the images themselves. You will loose all edits and make a big mess.
My guess is that once you get all your catalogs into one the missing folders will appear. I'm guessing they are just not in the catalog you think they are.
Users report single catalogs with hundreds of thousands of images without performance issues. If you are having performance issues I would look elsewhere to solve them.
I created a new catalog an tried to import the other one. It said that the folder from the last year would still be not be imported.
So I did the same thing with the older catalog from my lightroom 4 which still exists. It said that that I have to update the catalog before I can import them. I checked yes and afterwards, I got an import window with the older files as well.
Well I haven't imported many photos yet after the crash. Maybe I try to create one catalog an reimport the older ones from LR4 and reupdate. Maybe all files will be in there when I do it again. Forget about why I have many catalogs. The strange thing is that in all catalogs the folders from the last years did not appear. It did not happen once, it happened many times.