I found that the links to my posts did not work.
I almost finished formatting this site, but found that none of the links to my posts were working. At first I thought that just the link might be broken so I tried typing the URL for the post into a browser, but there this didn't work either. I don't know why, but I could go to the posts from the calendar.
I tried to fix it and did so many things, but could not get the links to work. In the end I decided to download a new theme since the problem seemed to be in the theme I use, WordPress Twenty Seventeen.
I found a theme called "Simplicity", it is free but has great support. It even provides "Simplicity-child", so I don't need to edit the original theme.
I downloaded the new theme and waited. I checked my site and it was fixed! I still needed to do some editing work, but I was so happy I didn't care.