Why did you change the motherboard?
From what old one to what new one?
Assuming a Windows PC, did you do a fresh OS install?
Have you swapped the cables?
Are you trying to connect via the motherboard HDMI (which means a CPU with integrated graphics), or the GPU HDMI port?
What GPU, if applicable?
I built a new pc because eveything In my pc was very slow and wanted a brand new one I got eveything new except the psu i haven’t got a gpu installed but my hdmi just ain’t working i have tried it on different screens and still isn’t working