These happen from time to time. It’s a malicious script that locks up your browser and as such your computer. It calls for urgency in calling a number. All these are, are pop up ads.
These can be closed out very easily with CTRL ALT DEL, choosing task manager, then force quitting your browser.
The way to avoid them is to watch more carefully where you click, being careful not to click on ads. You can also install AdBlock Plus to reduce the risk. https://adblockplus.org/