Take a close look at the memory retaining clips on both sides of the memory module and make sure they're fully engaged.
The retaining clips should look just like they did before you removed the RAM. They should both be in the vertical position and the small plastic protrusions should be fully inserted in the notches on both sides of the RAM, as shown above.
If the retaining clips are not fitted properly and/or the RAM will not set in the motherboard slot properly, you have installed the RAM the wrong way or there may be some kind of physical damage to the memory module or motherboard.
