Cannot open this spreadsheet with this message. It says Format error discovered in the file in sub-document content XM…
Hello - this sounds like a corrupt file and there is no default procedure to get this fixed, if possible at all. The only choice - as far as i can see - is to upload your file and then may be someone could investigate and fix. But may be you would need to turn to your backup file (Hopefully you got a backup).
Could you please upload and provide some additional basic information about your operating system, its version, your LibreOffice version and the format of your spreadsheet (.ods, .xls, .xlsx, …)?
Please use edit to update your question and do not post an answer. To upload your file, use the clip symbol in edit mode