It's not free. I presume that you haven't already entered the key (else you wouldn't have this problem) so you're stumped. You'll have to buy another copy of Excel. It's a bit like losing $100 note - these things are valuable.
IF you can not find the key you will need to either buy a new version to get a key, Call Microsoft Support to see if they can retrieve it for you (if you registered it or it is installed on the system they may be able to tell you how to retrieve it) or Get Open office -- It is a free program that can do everything that MS office does including open and save excel spreadsheets.
How to locate the product key for a full version
You may be able to find the product key for a full version in more than one location. This depends on how it was obtained (CD/DVD, downloaded, subscription, or preinstalled by the computer manufacturer, system builder, or reseller). To find the product key for a full version, check the following locations:
* Sticker on the CD case or other packaging.
* Certificate of Authenticity.
* Confirmation e-mail message that was received from an online purchase.
* Retail perpetual product key, located inside the new Product Key Card available at retail stores.
* Retail perpetual product key, purchased online from Office Online (http://office.microsoft.com) with the full download of Office.
If you still cannot find the product key for your full version, visit the "What to do if the full version product key is damaged or missing" section.
What to do if the full version product key is damaged or missing
If you cannot find your product key, you can obtain a new one. To do this, contact Support. Find the Support telephone number for your area by running the Product Activation Wizard on your computer. For more information about how to run the Product Activation Wizard, click the following article number to view the article in the Microsoft Knowledge Base:
950929 (http://support.microsoft.com/kb/950929/ ) How to contact a Microsoft Product Activation Center
When you contact Microsoft for a new Office product key, please have the following items available for the customer service representative:
* The product CD-ROM.
* The readable parts of your original product key (if available).