This could be caused by several things. Here are two things you can try.
First thing you could try is to reset the PRAM. Turn off your machine and after pressing the power button, press and hold the following 4 keys: Command, Option (alt), P and R. Hold those down until you hear the chime twice, then release.
If that doesn't fix your problem, you could try reseating the RAM modules. Make sure you're not statically charged by touching a metal surface when handling the memory modules. They're located right under your bottom case, which is attached with 10 screws. Only do this when the machine is turned off, make sure the magsafe is not connected and preferably disconnect the battery.