If you have a custom build, you'll need to find your windows 7 CD case, it has the key. If you have a pre-built pc, the key is on a little sticker located near the back of the pc. If you have no sticker and no CD case, you'll need to contact Microsoft Support, I'd suggest a phone call, and they should be able to help fix the issue or activate Windows with a new key. Either way, download Speccy afterwards and use that to look up the key and then Write It Down ;p