What to do if question mark appeared when i turned on my macbook?

Good day! I tried to downgrade my macos from big sure to catalina with the guide from youtube but then after erasing the macintosh hd files and try to restart my mac it didnt on then i tried to click the power button but the question mark appeared with the word under says “support.apple.com/mac/startup. What will i do to on the macbook and to continue what i started?