remove/disconnect all extras (video card, sound card, network card, hard drives, etc...)
remove one of the DIMMs and only leave one installed in Slot 0
hardware reset your BIOS (jumper, etc..)
Try to see if your rig posts (starts up).
If it doesn't try switching the other memory chip. Make sure your CPU is properly seated and it doesn't have any bent/broken pins.
Also make sure that the power needs of the motherboard are met. make sure all the connectors are connected for power (there may be several, check your manual).