If and when you need to resuscitate a Mac that won't boot up all the way, there are some commands that will save you a lot of time.
If your problem is not a true hardware issue then chances are there are disk errors and so the first task is to check and repair the disks. You can boot off an install CD/DVD or one with repair tools on it, but if you are comfortable with booting UNIX machines then skip that and...
Boot in Single User Mode
Hold down Command-S when you turn the machine on. You should get a gray screen which will quickly turn into a classic UNIX console with all sorts of cryptic text. At the bottom of which should be some instructions to either check the disks or to reboot in 'full' Single User Mode.
Disk repair with fsck
Your first step should be to run fsck, the UNIX disk checking program, with the options -fy. The 'f' forces fsck to check all available filesystems and 'y' answers Yes whenever it would ask you to approve a fix. If you skip the 'y' then you'll be sat there for hours responding to prompts.
# fsck -fy
That will likely produce lots of scary messages about different files. Don't sweat the details, just let fsck do its thing. Any files that it breaks were already broken by the underlying disk issue. fsck won't compound the damage, only repair most or all of it.
When it finishes, run it again! There can be dependency issues with disk fixes so to play it safe just run it multiple times until it reports no new errors. Typically the first run will get them all.
If you want to poke around on the filesystem then you can mount it with:
# mount -uw /
Reboot the System
If you are feeling lucky then just type 'reboot' at the prompt to see if your system comes back all the way. Fingers crossed...
If it doesn't then just restart the machine with Command-S as before and get back to Single User Mode.
'Full' Single User Mode
At this point you are in Single User Mode but with only the basic services. You can start up a bunch more stuff with
# sh /etc/rc.d
You'll see a bunch more verbage on the console. Keep an eye on it to see if any services are not able to start up - important clues to your problem. You'll either get a system prompt or an error. Hit return to get the prompt if you don't see one.
Now you have access to pretty much all the command line programs, man pages etc. an you are free to do untold damage to your system...
One useful program to know about is diskutil, which is the Mac tool for messing with disks. Look at the man page for details.
To see what disks the system knows about:
# diskutil list
To eject a CD/DVD in the drive (your device name may be different)
# diskutil eject /dev/disk1
Zapping the PRAM
You'll see mention about zapping the PRAM memory in the machine by starting it up while holding down Option + Command + P + R (don't worry about Shift for the P and R). Hold them until your hear two chimes.
This is easy to do - and has never had any effect on any system that I've played with...
Booting from a CD/DVD
To force a boot from a CD with recovery tools on it, turn the machine on, pop the disk in and hold down the C key. You should get to a gray screen with the spinning indicator. Let go of the C key and you should get an OS install menu, or whatever you have on your CD.
If a simple disk repair doesn't fix the issue then you may need to reinstall the OS from disk. Not a problem but be sure to select 'Archive and Install' in order to keep your existing files. Some might say just go for a reinstall regardless of the problem - but I would strongly suggest doing a disk repair first.
Good luck with your fix.
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