Graphics Problem in Numbers

I haven't been using Numbers very long since Excel refuses to save documents anymore. So I have to use the help guide a lot. Somehow in a spreadsheet I was editing, some cell borders disappeared. I am not aware of what I did to cause it. I can only say that I have tried everything in the help menu that has anything to do with cell border lines and more and nothing has worked so far. I cannot find anything else to try, so I thought I would put out a request and maybe someone would know how to get the borders back.

I'm using a fairly new iMac running 15.3.1.