Make sure that it is not your monitor, and if it not, then reseat everything. The biggest cause of no POST at all is poorly seated memory. After that do the diagnostic in these steps-
1. Test the power supply.
2. Try to barebones POST. This means stripping the PC down to only a single drive, a single stick of RAM, a CPU, motherboard, and some sort of graphics adapter. (use the onboard if you have it) Even unplug all USBs and front panel connections. Use a jumper to jump the power button if you do not have a reset button on your motherboard.
3. Swap out the RAM for a stick that is known to work.
4. Swap out the graphics card if you are using one.
5. Barebones "outside the box". This means try to to make the aforementioned barebones setup POST outside of the case. Motherboards that are poorly grounded to the PSU or case can keep you from posting.
6. If all of the previous things fail, it is ok. The PC is now disassembled and the motherboard is all ready to RMA or toss in the trash.
Now just hope it is not the CPU. Either way, I never just swap out a motherboard or CPU. As a rule, I always do them together.
BTW, I am a professional PC repair technician, I work in a real nuts-and-bolts shop and do this stuff all day, every day.
Hope this helps and good luck.