Numbers no longer opens .xls spreadsheet I have very recently converted to a MacBook Air from a PC and have a 1.3mb .xls file which 1 week ago Numbers happily opened, edited and saved in its own format. 

Today, when I opened it it was corrupted. So I have been trying to open the original .xls file again and the Numbers message is simply 'the file will not open'. No explanation, no clues as to why.

I can still open another (very much smaller and simpler) .xls file but don't know what to do next...
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.