Your question makes me wonder what sort of software you're using to try to open it.
Microsoft Excel used to use the file extension .xls until 7 or 8 years ago when they upgraded to XML format (Google it) and from that point changed the file extension to .xlsx.
You might have trouble opening it from a very old computer, a portable device, or perhaps even a Mac (though I should think Apple provide compatibility these days).
What operating sytem are you using?