my link tool has stopped working

I don’t get it. My link tool does not work anymore. When I highlight the word or phrase that I want in my post, then click my “link” button, the whole page turns dark (like looking through sunglasses) and everything freezes. The only way to get back to normal is to leave the page. This never happened before. Should mention I am on Safari on a Mac, but I tried my Firefox browser and the result is the same. What’s happening? Did I turn off a setting by mistake?