What version of Excel do you have? Usually that error comes from Excel 2002. You can upgrade to 2003 (MS Office 2003) at Amazon for $50 or less, and then download the free 'fileconverters.exe' from Microsoft, which enables an older MS Office to read the later files.
I know this, because it happens to me all the time, what happened to you. So finally I took the plunge and upgraded to MS Office 2003, on all my machines.
The upgrade will accept all your customizations, macros, etc. so the interface looks the same, post upgrade, because it will import all your Normal.dot settings. Difference is, that 'fileconverters.exe' will work in 2003, but not 2002.
.xlsx is Excel 2007 and later (secure, the extra 'x') format. So I don't see how you'd get that error if you're really using 2007. The only other possibility, is that someone password protected it, yet somehow you're not getting the password prompt.
I guess it's also possible that the file was generated by some other later version of MS Office which prohibits 2007 from reading it, but I can't see why. This is just a very wild guess.
Edited by brainout, 10 July 2015 - 10:52 PM.