Check your computer for viruses immediately! I had the same thing happen to me, only a different website. If there is a phone number in the pop-up do not call it! The phone number is for a place in India and they will try to scam you!
If you have been infected with malware (virus) it's best that you scan your computer with the built-in antivirus package and another one as well.
So scan your computer with whatever you have installed.
When that's done, load up Malwarebytes and do a second scan. You can download the Free edition from www.malwarebytes.org There is a trial, but run the free one instead...
Let it update and upgrade its self if necessary and scan your drives. It will clean up stuff quite well; just follow the on-screen instructions on quarantining and rebooting, etc.
