Why has one of my CAL sheets disappeared TWICE ? I have lost a lot information TWICE. Something about being related to Win Office 2003 ?

I am severely disappointed with this arrangement, TWICE I have lost a tabulation of my running Budget, for a couple of years. NOW it’s all gone. I have seen messages about having saved it in Win 2003 Excel format, which was an option, but I stopped doing that and perhaps that was a mistake. This is a travesty for me, what the hell happened to my Spreadsheet ?

With what file format do you lose you data?