Numbers no longer opens .xls spreadsheet
I would say both files have somehow become corrupted and Numbers cannot recover from the errors.

I would try to restore the original file from a TimeMachine backup if you have one. If not this would be a good time to set up a TimeMachine backup, very very easy to do with a Mac and OS X. 

I would also try opening the original xls file in OpenOffice or LibreOffice both of which are free.