How do I start up a Mac when all it shows is a Folder with a Question Mark, even with a DVD in the drive?
Image courtesy of Loïc Wolff
I just went through this my self!
Your Mac can't find anything to boot from. Or more specifically, it can't find the system folder on your primary boot device.
Try booting from your OS X DVD and running Utilities -> Disk Utility to check your boot drive for errors.
To boot from your DVD, insert in to Mac, turn off Mac, and hold C while you turn it on. You have to hold it until it chimes IIRC. That'll tell it to boot from the DVD.
A folder with a question mark means your Mac can't find the system startup software.
Follow the troubleshooting steps in this Apple Support technote: http://support.apple.com/kb/TS2570
I find both the answers from Ian C. and ghoppe correct and accurate, so I gave them both a +1, because they're both right.
If it was me I'd try the Disk Utility tactic first because you may have nothing more than a case of index corruption on the hard drive and that might fix it up. The next most likely cause, in my opinion, could be that the drive has developed bad sectors in the boot code and it can't even read the startup stuff. Finally, it's possible that the NVRAM/PRAM has become corrupt. This can happen, but not as frequently as the other two. I've seen indexing problems induced into Mac's when the power gets cut on and off quickly during the boot process. If this has happened to you it might be a good possibility, and Disk Utility might be able to clear it up.
If you use Disk Utility and it does a semi-recovery of your HD, it might be rendered read-only, meaning it can't completely reconstruct your index files, but what it can recover will likely be rendered read-only. In this case, a tool such as Disk Warrior may be of use to you, but it's expensive.
If you have an external hard drive that's bootable, you could obtain Scannerz (http://www.scsc-online.com) and scan the drive for bad sectors. Scannerz can also, if used skillfully, determine other problems on your system but it requires a bit of knowledge as to how a system functions. I could teach my dog to do basic drive testing on a drive with Scannerz, but using it to isolate other faults, like bad cables, connectors, or logic board problems requires at least a little knowledge about basic systems and hwo they work. The company that makes this product has some troubleshooting procedures about the process in their downloads section of their web site.
If your a hard core techie, once again with an external hard drive, you could download smartmontools (http://sourceforge.net/apps/trac/smartmontools/wiki), run it off the externally bootable HD and see if your drive is even working. Both Scannerz and Disk Utility only report what I'd call "threshold" S.M.A.R.T. data (i.e PASS/FAIL) but this can report a lot more. Unfortunately, S.M.A.R.T. can often miss some problems with a drive until an actual write operation occurs and fails, so problems may exist that such tools may miss. smartmontools is command line only and needs to be run via a terminal. Newer versions of Disk Utility also provide better S.M.A.R.T. reporting, but IMHO it's still not stellar.
With luck, it will be something simple.