Accidentally updated Sierra hackintosh in dual-boot environment, any way I can undo it?

Hi,

So, my friend was using macOS in my dual-boot environment with Windows (separate SSDs) and he accidentally pressed on 'Update after Restart' on the Update notification. I'm guessing the update didn't happen immediately since it basically happened the next day, but yeah, this broke it and macOS won't boot anymore.

Is there any way this can be undone, or do I have to just reinstall everything?

Thanks!