Password protecting an Excel document is relatively simple: How to Password Protect a Microsoft Excel 2003 Document.
As I understand it, recovering the password is no easy task. Recovering the file itself is far easier…but without the password it won’t be too much help to you. There are a few different versions of security used when protecting files, depending on what’s actually being secured.
Even so, you might want to read this article to get a better idea of how to move forward with your inquiry: Guide to Recovering Protected Microsoft Documents.
Hope this helps!