Pop-up saying that my computer has been hacked

I hope you didn't allow them remote access. An ad blocker will help a little. Best to note the sites it is happening on and stay away from them.

It's a common scam that has been around for a few years now. Click the X to close the window. You should then see a box at the bottom, Do not allow this site to create new messages. Check it and then close that window and finally the popup.

If that doesn't work, for Edge, follow the instructions by Scott_Ki here:
https://answers.microsoft.com/en-us/protect/forum/protect_other-protect_scanning/edge-frozen-with-virus-warning-popup/7212a393-90ad-48d0-ab55-4ffe1f21a554?page=2&msgId=7618ae67-bf16-4155-bf90-d8a7405ee199   

For all other browsers, use the Task Manager to exit the browser. Reboot and see if the pop-up goes away.

How to Use the New Task Manager.
http://www.wikihow.com/Open-Windows-Task-Manager  

If the problem persists, for any browser:

Remove Tech Support Scam pop-up (Call For Support Scam)
http://malwaretips.com/blogs/remove-tech-support-scam-popups/   

Please read:

PSA: Tech Support Scams Pop-Ups on the Rise
https://blog.malwarebytes.org/fraud-scam/2014/11/psa-tech-support-scams-pop-ups-on-the-rise/   

Breaking down a notably sophisticated tech support scam M.O.
https://blogs.technet.microsoft.com/mmpc/2017/03/02/breaking-down-a-notably-sophisticated-tech-support-scam-m-o/   

For more information, see this post by GreginMich
https://answers.microsoft.com/en-us/protect/forum/protect_other-protect_scanning-windows_10/my-computer-was-blocked-how-do-i-unblock-it/4735ea26-4e01-40eb-9c34-c991d11ad194#LastReply