Seems like your Excel file got corrupted. Try to save as .xls (without the extra "x" at the end). Then re-open and convert to .xlsx again.
It might be some lay-out and format definitions are gone, but it also may just do the trick.
Also on this forum some valuable tips have been shared before on how to deal with corrupted files. As there are many possible causes, from the (generic) error screenshot it is hard to tell what could be the cause in your specific case.