Not all steps may be required to resolve the problem, you may be able to simply reboot into Safe Mode and reset NVRAM and fix the white screen issue, for example.
Reboot into Safe Mode
This is quite easy; simply reboot into Safe Mode on the Mac with a properly timed shift key press.
Reboot the Mac as usual, then immediately hold down the SHIFT key, release the SHIFT key when you see the Apple logo and progress bar
Attempting a Safe Boot is easy and it can remedy some simple issues with a Mac getting stuck on a white screen during boot. If the Mac works fine in safe mode, try rebooting again as usual (without holding down Shift) and see if it works as usual, it just might.
You can learn more about Safe Boot mode on Mac here if interested.
Often simply resetting NVRAM / PRAM is sufficient to remedy a white screen Mac issue:
Reboot the Mac, the moment you hear the boot chime, hold down the Command+Option+P+R keys concurrently, when you hear a second boot chime you can release the keys, the NVRAM has been reset
After NVRAM has been successfully reset, go ahead and let the Mac boot as usual. At this point it should no longer get stuck on a white screen.
Since the white screen problem seems to occur mostly on portable Macs, we’ll focus on resetting SMC on modern MacBook Pro, MacBook, MacBook Air models:
Shut down the computer and connect it to your MagSafe adapter and a wall outlet as usual
Hold down the Shift+Control+Option+Power button at the same time for a few seconds, then release all keys concurrently
Boot the Mac as usual
For other hardware, you can learn how to reset SMC on Macs here.
Reboot and Repair Disk
If possible, try to repair the boot disk through Disk Utility by entering into Recovery Mode:
Reboot the Mac and hold down Command+R to boot into Recovery Mode
Choose “Disk Utility” from the list of options at the Utilities screen
Select the hard drive and then go to the “First Aid” tab and choose to run First Aid and repair the drive
If the drive is showing many errors, particularly errors that can’t be repaired, you may have an underlying issue or drive failure impending. If that’s the case, be sure you backup your data from the Mac, and then consider using Apple Hardware Test to diagnose any issues. Often replacing a hard disk is in order, some users are comfortable doing it themselves otherwise you might want to contact an Apple Support center for official assistance in replacing the drive or diagnosing whatever other system issue persists.
Boot with Verbose Mode
This won’t doesn’t make a lot of sense, but it sometimes works: boot into Verbose Mode. Keeping in mind that all Verbose Mode does is detail what’s going on during system boot, similar to watching a linux machine start up, it’s not entirely sure why this works but there are multiple reports on Apple Discussion Forums that it does.
Reboot the Mac as usual, then immediately hold down COMMAND + V keys
Again, it’s not entirely clear why this works, perhaps it’s just the general act of rebooting yet again as verbose mode shouldn’t really do anything different aside from appearances, but nonetheless it does seem to sometimes work to bypass a stuck white screen on some Macs.
Reinstall Mac OS
If the other approaches are failing, you may need to reinstall Mac OS system software. You should always backup your Mac before doing this. Aside from having a proper backup, reinstalling Mac OS or Mac OS X is fairly straight forward: