Can you close the pop-up or window?

Yes — I closed it

Close all browser tabs and applications, run an anti-malware scan, take any suggested actions, then restart your computer.

No — it won't close

Step 1. Force quit the browser:

Step 2. If that fails, briefly press the power button to shut down. If it still won't respond, hold the power button to force off.

Step 3. Power back on, run an anti-malware scan, take suggested actions, then restart again.

Never call a phone number shown in a pop-up — these are scams.