Why does my computer randomly need a product key?

The other day i went to turn on my computer and after i started it up and window came up with a black screen in the background. It said something about not being able to use the product key anymore and i need to either buy a new one or enter a different one. i have an option to activate it or do it later...when i click do it later my computer comes on with a black background and writing in the bottom right hand corner saying this copy of windows in not genuine. I bought my laptop from a store a few years back so i dont know why this is coming up. Its an HP pavillion laptop. i dont know if that helps;.