Mac doesn't boot
From Mac Guides
I decided to write this because there are several threads about this posted everyday and I got tired to write the same things to do over and over again. I hope this helps but I cannot guarantee that this will fix your issue. Please, remember to back up frequently.
What does it do?
Can you hear the fans and hard drive and see the white boot screen? Flashing question mark? The circle just runs and nothing happens? Only black screen and nothing else? These can be very helpful to diagnose the source of the problem. 
- Flashing question mark : This is usually a sign of a failed hard drive. This indicates that boot up volume can not be found.
- Black screen: This is usually a sign of a failed logic board. There is no single reason for this and this may be caused by failed GPU, screen, CPU or RAM.
- Circle of death/no circle/blue screen: This is usually a sign of a hard drive failure. This may NOT be a dead hard drive, just broken permissions or something
- Gray screen/Apple logo/spinning gear or globe/prohibitory sign/folder with flashing question mark : This could be a sign of faulty hardware but more likely a software issue, i.e. broken permissions, failed software update, etc.
What to do
First, hold down the option key (also known as the alt key) just after you've pressed the power button. This should show all the available options to boot from, so if you have Windows installed through Bootcamp or a disk in your SuperDrive, you should be able to see them along with your OS X drive/partition. Select the OS X partition with arrow keys and press enter. Startup key combinations
If that doesn't help continue doing the steps from below:
1. SMC reset . Shut down and unplug all cables including power cord for at least 30 seconds and then plug in power cord and keyboard/mouse.
2. PRAM reset . Hold down option, command, P and R while booting. Start it when you hear the sound and don't let go before it reboots.
3. Safe boot . Hold down shift while booting to boot into safe mode. If this works, open Disk Utility from Utilities and repair your permissions and verify the disk
4. Hardware test . Insert the install disk (disk 1 in Mac which shipped with OS X 10.5.4 or earlier, disk 2 if shipped with OS X 10.5.5 or newer) and hold down "D" before the grey boot screen opens. Follow on screen instructions
5. Boot from install disk. Boot from the disk that came with your Mac. Hold C while booting. Select Disk Utility from top bar and repair permissions and verify the disk. Reboot normally and see if it works now.
6. Boot from another Mac (Firewire target disk mode). Connect the machines with a Firewire cable. Power on your Mac and press T. Select the other Mac's hard drive. Download the current OS X combo update. Run the combo update installer and select the drive that won't boot as the destination.
7. "Repair" OS X. Boot from the install disk as in point 5. and reinstall OS X. This should NOT erase any data, just replaces the system files which may have caused your booting problem. Now install the latest combo update for OS X and repair permissions on your drive with Disk Utility.
7.1. Install OS X into an external hard drive and then boot from it  and try to copy your data from the hard drive before proceeding. If you have done a backup recently, you don't have to do this.
8. Restore OS X from Time Machine backup. Boot from the disk that came with your Mac and choose the Time Machine restore option. This step will lose any changes made since your last backup but the system and your data will be put back to exactly as it was on that date.
8.1 Reinstall OS X. Boot from the install disk as in point 5. and select Disk Utility again. Now erase the whole hard drive and install OS X again. If you had a Time Machine backup and step 7 worked you don't have to do this.
- If you still cannot boot properly, you should contact Apple(Care) if you haven't already. It's very likely a hardware issue if none of the above helped.
- All Macs come with 1-year warranty and 90-day phone support
- Warranty and phone support can be extended to 3 years by buying an AppleCare
Replacing parts yourself
- If you have run out of warranty and don't want to pay Apple to do it, you can do it yourself as well. Please make a thread about your problems if you haven't already so we can tell you what needs to be replaced and recommend what brand to buy and so on.
- Replacing some parts on your own voids warranty while some don't (e.g. RAM and hard drive). Make sure you are out of warranty when you decide to replace something yourself.
- There are great tutorials of e.g. how to replace hard drive in Youtube and iFixit  for example. iFixit has also a part store where you can buy replacement parts for you Mac.
--Hellhammer 08:14, 6 March 2010 (EST)