What system are you running? Have you updated your system software?
Try each of the following steps in this order before you reset the SMC. Test the issue after completing each troubleshooting step to determine if the issue still occurs.
Press Command + Option + Escape to force quit any application that is not responding.
Put your Mac to sleep by choosing the Apple () menu from the upper-left menu bar and then choosing Sleep. Wake the computer after it has gone to sleep.
Restart your Mac by by choosing the Apple () menu from the upper-left menu bar and then choosing Restart.
Shut down your Mac by by choosing the Apple () menu from the upper-left menu bar and then choosing Shut Down.
If your Mac appears to be running normally but isn't responding, force the computer to shut down by pressing and holding the power button for 10 seconds. Note: You will lose any unsaved work in any open applications.
For Mac portables experiencing issues related to the MagSafe power adapter and/or battery, try:
Unplug the MagSafe power adapter from the Mac and the wall power socket for several seconds.
Shutdown the Mac. Physically remove and re-insert the battery (if it's removable) and startup the Mac.
If the issue is still not resolved after following the troubleshooting items above, an SMC reset may be necessary.
After performing normal troubleshooting, these symptoms may indicate that an SMC reset may be necessary:
Resetting the System Management Controller (SMC)
Resetting the SMC on Mac portables with a battery you can remove
Note: Learn about removing the battery on MacBook and MacBook Pro.
Shut down the computer.
Disconnect the MagSafe power adapter from the computer, if it's connected.
Remove the battery.
Press and hold the power button for 5 seconds.
Release the power button.
Reconnect the battery and MagSafe power adapter.
Press the power button to turn on the computer.