my Safari is blocked by a pop up

As you are probably already aware the popup is a scam. Just close the page in the usual manner.

If you are unable to close the page, or if it persists upon opening Safari again, proceed as follows.

Quit Safari. If necessary, force Safari to close by following these instructions: Force an app to close on your Mac - Apple Support.

• Summary: choose  (Apple menu) > Force Quit...
• Or, using three fingers press the three-key chord ⌘ (the Command key, next to the space bar) Option (the key next to it) Escape (the key at the upper left of your keyboard or Touch Bar).
• A dialog box with the title Force Quit Applications will open.
• Choose Safari, click the Force Quit button, and confirm the dialog with Force Quit again.
• Close the dialog box.

Press and hold a Shift key and keep it depressed while launching Safari again.

• When Safari opens, release the Shift key.
• This action prevents Safari's previously loaded pages from loading again upon launch.

If you are using an iPhone, proceed as follows.

Force Safari to quit by quickly double-clicking the Home button. On that screen, swipe left or right until you find Safari with a preview of the problematic web page. Swipe that image up and away to terminate it:

Force Close apps

The unresponsive Safari page will be gone, but if you were to launch Safari again it might just reappear. To prevent that from occurring, go to Settings and scroll down a bit until you see Safari. Tap Safari, then tap Clear History and Website Data. Confirm the dialog that appears next, and you'll be back in business. The effect of clearing website data will require you to "sign in" again to websites that require authentication (such as this one).

For an expanded description of this and other common scams, read Phony "tech support" / "ransomware" popups and web pages - Apple Community.