Just contact MS or check under your laptop for the serial.
There are many possibility's maybe it's because of Update. Second thing is that maybe someone else is using the same key on other pc and maybe then it's been blocked by microsoft.
If you bought your laptop from a retail outlet and it comes with win7 preloaded then just call microsoft for support, otherwise as elvin said use an activator.
You may have had your serial stolen by a piece of malicious software and then leaked just an heads up.