Motherboards usually come with some way to communicate an error message to you, either via beeps from a onboard speaker, flashing led lights, or a fancy analog display to display 2 digit number codes. You can use this to determine what part of your system is causing the issue.
However, you can also find out through trial and error. First, remove the CMOS battery for a few minutes to reset the bios. Try re-seating your ram and attempt to boot up again. Poor contact with the slot and the ram may sometimes be the issue. If that doesn't work, choose to keep just one module of ram and try out all the different slots until they system boots. Swap it out with the other module if it still doesn't boot.
Failing that, try plugging in a display to the HDMI port instead of the DVI port, and see if it POSTs.
The final feat of desperation, try to re-seat the CPU, boot it up, and hope it does the trick.
If nothing seems to work, you might have either a dead motherboard or a bad set of ram on your hands. Of course, nothing is absolutely certain at this point either. If you had a second working system to test the ram, you can rule out at least the ram be faulty. And if anything is presumed to be dead, you can always go back to the retail store and have it exchanged.
As a side note, it would be best to buy some thermal compound and rubbing alcohol(99% Isopropyl alcohol ) to properly reapply the thermal compound. Simply slapping on used thermal compound can produce air gaps and prevent efficient transfer of heat from the cpu to the heatsink.