White screen With Apple logo

Reset SMC and PRAM .

1.Try a Safe Boot

Mac OS X 10.2 and later Safe Boot includes disk checking and repair. First start your Mac in Safe Mode. To do this, shut down your Mac. Now Turn on your computer by pressing the power key while holding down the Shift key. When you see the Apple logo, release the Shift key. After your Mac fully starts, restart your computer normally without holding any keys/buttons during startup. Note that Safe Boot is slower to boot.

2.Run Disk Utility

Turn your Mac off by holding down the power button for a few seconds. Turn on your Mac and hold down the Command and R keys until you see the Apple logo. Then you should see a “Mac OS X Utilities” window. Select Disk Utility, click your OS X hard drive and select verify/repair disk.

3.Reset PRAM

This may fix your boot failure. PRAM (“parameter random-access memory”) stores some of your settings. You may reset PRAM. To do that, turn off your Mac. Now turn on your Mac. Then, immediately, hold down the Option-Command-P-R keys until your Mac restarts. When your Mac is starting up, release the keys.

4.Reinstall Mac OS X

If nothing above works, you may try this. Your Mac needs to be connected to the Internet. See number#2 above. Follow the same steps. Restart your Mac and hold down the Command key and the R key until the Apple icon appears; when you see the Mac OS X utilities window, select the Reinstall option.