Not booting anymore after changing mobo

I had a hardware fault that I tracked down to being either the motherboard or the CPU, so I decided to just replace both. (And with that I also had to replace the memory.)
My system had been set up with a dual-boot for win 7 and win 10, which IIRC I set up using EasyBCD.
After replacing the hardware, I still managed to boot into win 10 just fine, and after replacing/updating several drivers it appears to working.

However, when I try to boot into win 7 I have problems.
It always prompts me with a "didn't run correctly" screen where it offers to boot into safe mode and all that, but when booting normally it gets to the "starting windows" screen, then it flashes a blue screen and jumps back to my BIOS's logo, and starts the boot process all over again.
I tried booting into safe mode, and (although I wasn't present at the moment it screwed up) it still just went back to the start of the boot process.

I'm not sure where the problem lies, nor how I should go about fixing it.
Is it more likely that my copy of windows 7 got screwed up somehow and it needs to be repaired? Or is this a problem with the dual-boot not working with the new bios, so I need to fix that? Is win 7 having some sort of innate problem with compatibility with this new mobo?

You didn't give us the hardware specs of the old and new MoBo.

New hardware requires some drivers that are not on the Win 7 files.
You may need to add those drivers (normally USB3.x, SATA and NVMe) to the Win 7 installation. From Win 10 you can use Dism++ to do it.
SIW2 is the one to help you on the task.