Download, re-installed Microsoft 365, but none of my Excel workbooks/worksheets work.

Last Saturday my Desktop was hacked and wiped. I have an HP Desktop Pavilion, so the 1st place I called was HP. Their Tech spent 3.5 hours recovering as much data as he could but had a problem with Microsoft Outlook. I called Microsoft and the Tech spent about an hour fixing the issues. We called it a day, Next morning I opened my daily Excel workbook to see a totally new and different format, none of which allows me to enter data. My data is in simple "Currency" format, + and -. This new workbook doesn't allow me to enter any data at all, plus it doesn't display decimal points, dollar signs, or red. So far 4 Techs from Microsoft and 2 Techs from HP have been unable to solve the problem. I really need some good Excel expertise and knowledge, and maybe a previous history of this issue.