The most common thing it will be is the RAM being not properly seated. If you're happy with doing it, open the case (with the PC turned off and unplugged), open the clips at either end of the RAM modules, pull out the RAM modules and then push them back in again. Then push the clips back in again. They should click into place. Make sure you touch the RAM as little as possible, holding it by the ends, and touch something metal like the case before you touch the RAM to discharge any static.