I have a MacBook Pro with Microsoft Office 2011. When I migrated from my previous computer, I kept Office on both computers, purchasing a key when prompted on the new computer. That was several months ago.
This morning I went to open a Word document a friend gave me, and it gave a small black window and asked for a product key.
I can probably find the product key in my email archives, but I basically wanted to know why Office suddenly decided that it needs a new key, and if possible how to tell it "I purchased and gave you the product key you wanted; please get back to work."
The product key is one I have only used on this computer, and it worked when I re-entered it.