swap the old stick back in. If the PC boots, then carefully check that you install and seat the new sticks correctly and that they don't bump against other components that prevents them from being seated normally.
Then boot again.
If you can boot into BIOS, you may want to check whether the BIOS sees the memory and whether you can/should adjust voltages slightly to get going. If you can't even boot into BIOS, then you should return the memory.