'No entry' sign on boot

Hi,

my hackintosh used to work, then I tinkered with it without a backup and here I am now reinstalling from scratch.

The error I got after changing some settings was the 'no entry' sign immediately after boot. Now I've built a new Mojave install USB and upon booting from it, I get the same 'no entry' sign!

Can anyone help me understand what this is, and how I can get around it?

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My build is a i5-6700, on a Gigabyte GA H170N wifi mobo, and intel graphics and onboard sound. It did all work (until I messed with it!)

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Thanks!