Things to try
1. reset the cmos. Instructions should be in the manual but it's usually moving a jumper for a few seconds and then moving it back. This puts the
CMOS memory to default
2. make sure the 24 pin connector is on the motherboard and the secondary power cable(4 pin?) is also plugged in.
3. Verify the reset and power plugs are correctly installed on the mobo.
4. Remove the CPU to make sure no pins are bent, then replace.
5. remove all drives and any other peripherals except for the video card. This includes the hard drive.
6. make sure all power connectors are plugged into the video card.
7. install a single stick of ram.
8. try to start. If it doesn't, power down and swap the ram stick for the other one and try again.
9 if it starts, shut down and install the other ram stick. Repeat, then do with each drive etc until all is running.
10. If it doesn't start, before RMA go to the manufacturers forums or FAQ's to check for common problems that might be causing the issue.
Good Luck