How to fix black screen

During changing my screen resolution for Fortnite, I downloaded this app which allows me to able to get a screen resolution which isn’t shown on the options. I didn’t read the instructions clearly and after setting it, it turned black. I tried to reboot or reinstall windows or whatever but it said I need a recovery key. It wasn’t my laptop at the time but my sister’s and she said that she didn’t set one and ignored it.