888-919-6138

That is a malicious Pop-Up. These things can get on your computer usually by visiting dangerous websites. Do NOT call that number. The scammers will try to get you to take remote control of your computer and then all Hell breaks loose.

You can google "How to get rid of Popups", but it is wise to know if you are actually getting rid of the things or getting into worse trouble. Get some advice from someone experienced in these things before you make any changes to your computer.

If you search under the exact message you saw, that can help narrow the fix down. Pop-Ups are annoying, but usually not harmful by themselves. The harm is calling the phone numbers or going to websites suggested by that Pop-Up.