i received an Apple platform security alert on my macpro and its now frozen. How do i get rid of this and unfreeze my screen?

It's a scam.

Try these in order until you get control back:

Close the current web page

Quit safari

Force Quit safari

Restart your mac

Power-down your Mac.

hold shift:

• at startup to not load any extensions and start in safe mode, which checks the disk as it starts up

• at login to not launch previously open Apps

• At launch of Safari or other browser to not open previously opened web pages, one of which was running that scammer's script that locked ups your browser.