Black screen after failed Windows installation on MacBook 2014

Hello everyone,

I’m using a MacBook 2014. I know it’s an old device, but it’s the only one I can afford right now.

I tried installing Windows using Boot Camp, but the process failed. After restarting, my Mac only shows a black screen with the text support.apple.com/mac/startup.

At the moment, I can’t access either macOS or Windows.

I’d like to ask how can I completely reinstall the operating system from scratch?

[Re-Titled by Moderator]

Original Title: Macbook problem