When I booted my computer this morning, all my LibreOffice spreadsheets had been converted to Excel spreadsheets. Some of these had originally been Excel files, others created in LibreOffice
The files haven’t been converted; your operating system (Windows, Mac?? - you haven’t specified) has had the file associations changed. Is it possible that MS Office has recently been installed?
Thanks for your reply - no MS Office has been on there for ages, but I no longer use it
So you’ll need to use whatever means your OS (still unspecified?) provides to set file associations such that your LO documents are associated with LO applications.