But the last time something went wrong. I see an error that the document has an unknown format. How can this be !? In fact earlier I opened this file without problems.
It sounds like your Excel has crashed before you have saved the changes you have made. If AutoRecover has been set, excel recovery will try the latest save it has, unless the crash occurred before the first autosave is made. You can make these happen according to a time interval set in your Excel options. For instructions on this see the link below: