Already burnt a bootloader, now it won’t work.

I’m using an Atmega328p-au. Using an arduino as an ISP, the bootloader was uploaded for a Pro Mini, 3.3V using an internal 8MHz oscillator.
All worked perfectly. Then uploaded a blink sketch, hurray! It worked. Uploaded my code for a project. Dint work. Tried lots of things but nothing.

My next plan of action was to reupload a bootloader for an external oscillator and see if thats the problem. Well, that was two hours ago. I’ve rewired, changed breadboards, the lot.

Then I read about reflashing, is this the case? Anybody point me in the right direction. Google is not playing ball.

I’m using Minicore, if that makes a difference.