There could be multiple reasons for a pc not to start up with a new motherboard.
You need to provide more info.
Connected hardware, PSU, RAM, GPU, CPU, HDD.... details.
Beeps, Lights, Sounds and even smells sometimes.
What have you tried?
As nice as it would be to look at your PC and be able to tell you what the problem might be, we can't, so specifics are important.