I don't have much to suggest unfortunately, and it sounds like the note got corrupted somehow. If you create a new and similar in content note, does it happen in the same way?The problem is that there was a lot of text and there were several columns. All the text has disappeared. But it looks the same, both on my Mac (Catalina) iPad (iPadOS 13.6.1) and iPhone (iOS 13.6.1)
Going with that idea a little further, does all of the content appear if you paste it into a Pages or Word document?If I make a new note, then there are no problems. Just tried to copy the text and threw it into a new note. Just the 5 empty columns come in. Otherwise a very good attempt, I had not thought of it myself.
You're most welcome. One more tip: for very important notes, I use the Notes share sheet and save it to Files. This saves the content as a text file, which is a good backup.But thank you very much for your help and your good solutions. That way, I can also say I HAVE tried.