Why has spreadsheet jumbled all my information?

For MailMerge, i use Mozilla Thunderbird and set up a spreadsheet with libreoffice with one column showing first name and the second showing mail and underneath inputting the respective details.

When I opened it in this past this worked perfectly but recently, all the first names and emails have been added into just one tab in one long list!

How do I return it to the original please?

What steps do you follow to get the issue. What file format do you use to save?