LibreOffice lost rows / accidental sorting

I have a LibreOffice spreadsheet file to which I add a new row at the end each day. The first column contains the date.

Yesterday when I went to add the new row, I saw that ten day's worth of entries were missing from the end, and thought that I had somehow accidentally deleted them. But undo was not available, so whatever happened must have happened the day before.

I thought the rows were permanently lost. Today I realized they had just been resorted by one of the other columns instead of the date.

To see what the rows are currently sorted by, select all the rows by clicking the top left cell. Then select from the menu, Data - Sort...
It will show the current sort keys, and will let you change them.