All of a sudden on an existing worksheet, when I try to sort by column, Excel
won't allow me to select the top row as my header row. The top row is a
frozen pane so that I have it as headers as I scroll down through the
spreadsheet.

The frozen row is not something I've done recently. I was previously able to
sort and when I check the use header column option, it automatically gave me
that row. Now it's picking the 2nd row.