I would try two things to start with:

Reset the SMC

1. Shut down your MacBook Pro

2. Plug in the MagSafe adapter and make sure it is charging

3. Simultaneously press the shift + alt + control + Power buttons and hold them for about 10 seconds. Let go after that.

4. Restart your Mac (and move on to the following suggestion):

Reset the PRAM

1. Press the power button to start up your MacBook Pro

2. Hold down the following keys:⌘ + alt + P + R until you hear the start up chime sound twice.

3. Let go and boot normally.

Test that out and tell us what happens.

Make sure you are using the laptop with the correct model magsafe charger - if you use a different model's charger, eg a 15 inch 85 watt charger or such, it can cause this problem to occur. A reset of the PRAM would normally cure the problem temporarily.