I'm sorry about the issue. That is a fake popup message; it is a scam. Don't call that number, and don't respond to any phone call that you do not know or that you are not expecting.
Can you please close that popup window, then see if you can use the computer?
If it does not let you close that popup window, can you please shut down / restart the computer, and check?
If it does not let you shut down the computer, please press and hold the power on/off button on the computer / device for about 10 seconds (if needed, try 30 seconds), then turn it back on and check.
If it still does not work, do you have windows xp, vista, or 7 or 8 or 10?
Please reply at the box below to continue troubleshooting. Thank you.