Sure the RAM is correctly seated in each slot and the retention clips are securely locked? Did you use the correct slots? The pair should be installed in slots with the same color retention clips. And you would install the pair in slots that are probably numbered DIMM1 and DIMM2 on the motherboard. Keep in mind that DIMM1 is probably the second slot from the CPU, not first, and DIMM2 with matching clips would be the 4th from CPU. One of the new pair should go in the same slot as the factory module, and the other one in the slot with matching clips. If that's all correct and it doesn't boot, try clearing BIOS: If it still won't boot, try each new module, one at a time, in DIMM1 in case you got a bad module. And re-seat BIOS each time to make sure the PC sees the change. If neither new module boots when tested alone, remove the new ones, and put the OEM module back into the correct slot to confirm PC still boots... EDIT: As usual [NAME] typed faster than I did... 
I got my new XPS 8940 desktop a couple of weeks ago.  It came with 8GB RAM but I needed about 16GB for the work that I do.  So after some research on the Crucial website, I purchased this RAM: [URL] Crucial Ballistix 16GB Kit (2 x 8GB) DDR4-3000 Desktop Gaming Memory (Black)BL2K8G30C15U4B When I installed this memory in the system, it would not boot.  I keep seeing the light flashing on the power switch.  2 Amber flashes followed by 4 white flashes.  Upon researching, found that this means faulty memory.  I have chatted with Crucial who insists that their database (for the compatibility matrix of desktops and memory upgrades) is accurate and this memory is compatible with the Dell XPS 8940. Has anyone had any luck getting gaming memory (rated at 1.35v) working with the XPS 8940 (or 8930) desktops?
XPS 8940, RAM upgrade issue with Crucial Ballistix RAM