Hello Jan,
If you originally received the Product Key thru email, then you will need to get access back to your email or use the "Contact Us" link (on the page that you ordered the Office Product key) to contact the group that you actually bought the key from.
Or
If you bought a downloadable copy of Office from http://windowsmarketplace.com/ then please check your Digital Locker for the product key.
Or
If you bought a Retail copy of Office from a store, please see the inside of the plastic box (that the Office CD came in) for the product key.
Thank you,
Darin Smith
WGA Forum Manager