1
Where is that notification appearing is it in a web browser and does your PC appear frozen and are many window opening and is there a phone number indicated to call to resolve the problem?
If that is what you are seeing, that is a well known scam
Open Task Manager (Ctrl + Shift + Esc), on the Processes tab, find your web browser and then right click and End Task on your browser.
That should resolve the problem, you can then continue to use your browser.
2
If that is not what you are seeing, please provide a screenshot of the malware notification you are seeing.