The original files (23.000) are on an external harddisk. In my new computer I have a 5TB harddisk, so I copied my original files tot the new harddisk. I downloaded a copy of Lightroom 6 and imported the original files and also the Lightroom Lrcat. Apparently something went wrong. I lost the collections and also unable to edit the files as I receive a message that Lightroom cannot find the original files. If there is a simple method to fix this problem, I will be very happy. If not, where can I find (in NL) a person who can solve this for me.
If you are getting the message cannot find the original files. I would suggest you to please reconnect the images in Lightroom.
Please have a look at the following article : https://helpx.adobe.com/lightroom/help/locate-missing-photos.html
Importing is never the way to move files that have previously been imported into Lightroom.
You want to copy the photos and the catalog file to the new computer, if possible, putting them in the exact same location on the new computer as they were in on the old computer.
Open the catalog file by double-clicking on it, and if necessary reconnect the photos in Lightroom via these instructions: Adobe Lightroom - Find moved or missing files and folders