Q: Help needed for white screen

A question mark at startup means the mac can't find it's operating files.

If a DVD is ever stuck inside, hold down the left click on your mouse/trackpad and turn the mac on.

To try and resolve the issue of booting, turn the mac on holding ALT and try selecting Macintosh HD.

What OS are you running? (EG 10.6.8, 10.7.3 etc.)

From the Apple () menu, choose About This Mac. The About This Mac window shows your OS X version number.

LINK.

Have you managed to boot the Mac yet? If so, but the DVD is still stuck in the drive here are a few more suggestions for ejecting it: http://osxdaily.com/2009/08/28/eject-a-stuck-disk-from-your-mac-dvd-super-drive/

Hold down the eject key when you hear the startup tone and continue holding it. This should eject the CD/DVD.

If the operating system is 10.8.3 as per your earlier post and you still can't start try booting into the Recovery HD - turn off the Mac and restart holding down Command + r and hopefully it'll boot. At the screen with four choices select Disk Utility and when it launches click on Macintosh HD. Click on 'repair disk' and if anything gets changed click on repair disk again until you get a clean pass. Once finished restart as normal from the Apple menu. Will it now boot?

Now that the disk is out of the drive does the Mac boot normally? If so, it's job done - if not, read on...

Not quite sure what's occurring - Command+Option+P+R is a PRAM reset, whereby you hold those keys at bootup until you hear the second chime, then let go and the Mac should boot.

I was suggesting Command+r which is the Recovery HD which should take you to a screen with four choices where you can select Disk Utility and try repairing the drive as suggested above. I don't think, at this point you need to re-download and install the OS which is what internet recovery is.

I'm guessing you held down Command+Option+r. So quit what you're doing and just try to get it to boot into the Recovery HD.

Does the Mac boot normally now before you try any of this, or does it still stick at the screen with the question mark?

To shut it off do you have an Apple icon top left of the screen? If so, select restart and when the screen goes black hold Command + r until it boots into the screen with four choices.

If there's no Apple menu (I've never launched internet recovery, so I don't know) you'll need to press and hold the on/off button until it shuts down.

OK, let us know if running Disk Utility on the hard disk helps. If it does find and correct any problems, click on repair disk again until you get a clean pass - then restart as normal from the Apple menu.