One of three things:
1. The RAM isn't seated properly.
2. One stick is faulty.
3. One slot is faulty.
Take out both sticks, put one in the bottom slot and boot the machine. If it boots, turn it off and move the stick to the top slot. If it boots then you know both slots are OK.
If, after you've put the stick in the bottom slot, the machine doesn't boot, move the stick to the top slot and try again. If it still doesn't boot then you've got a faulty RAM stick. If it boots in the top slot but not the bottom slot then something's gone wrong with the computer.