Set computer to pc only and now my monitor won't work please help

 I'm using Text-to-speech because it's a lot to explain. And I'm really confused, but long story short, I had to download a secure browser to take tests for my new school, and it said that it wouldn't work with more than one monitor but I only have one monitor and Google said that I need to set it to just one. So I did the windows key plus P and did just display on PC monitor or something like that. And now my screen is black, and I don't know what to do. Please help.