Cannot open an Excel spreedsheet.

Version: 2008
Operating System: Mac OS X 10.5 (Leopard)

I'm a new Mac user, so please beare with me. I have the mac 10.5 with the
2008 office. Last night I was working with an excel spreedsheet, when
finished working I close the program and computer correctly.

Today I've been trying opening the same document, but I get the following
message: "Excel cannot open this file. The file might have been damage or
modified form its original format".

Through finder I can look into the contents of the document, but can't
access the information.

My question are the following:
1. How can I repair the document?
2. Is there a backup document?
3. Is there any other way to fix the problem.

It's is im****tant to me that I can fix this problem, since I've been
working on this document over a month.

Thanx