Did you try booting without a GPU? It should fall back to the internal GPU. Also there should be a jumper next to the BIOS battery, usually it's labelled CMOS reset. Switch its position, turn power on, then off and put it back in its original position. Voilá, BIOS reset. If there's no jumper, take out the battery and wait a few minutes and the BIOS will also reset to default.
I also second JayDDees recommendation about testing with a different GPU if available or testing the GPU in a different mainboard. But I wouldn't rule out power, these mainboards are weird af lol.
Cheers and good luck!