My computer has been blocked

Do not call them. It's a common scam. Click the OK button at the bottom of the popup. You should then see a box Do not allow this site to create new messages. Check it and then close the window.

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/1-888-2361-scam/0990122e-61a9-431b-947e-a904c202fa7c?msgId=8d26f1a8-041a-4351-9c24-a222fb305cbd