I thought it was the one on the Splash Screen when you open an office
product, but it isn't - I've just checked. It's the one on the CD case that
contains Office and consists of a series of letters and numbers and which I
guess, from your message, has been lost.
This link tells you how to get a new key by phoning microsoft.
http://office.microsoft.com/en-us/help/HA011508341033.aspx
but I can see limited usefulness here, they won't help if you are trying to
re-install your OEM version of Office after a crash but have lost the CD
case - easy to do as some of them come in a flimsy envelope rather than a
case.
There are a number of Product Key viewers available online, eg.
http://www.magicaljellybean.com/keyfinder.shtml
some claiming to be freeware and some not very expensive but I've never
tried them so I don't know how reliable they are.
Evi