Ask Max: eMac will not fully boot up after OS update
First of all thank you for this service, number one in my book. Here's my problem... I downloaded update material w/out checking what it was. Come to find out it was OS X 10.4.9 for PowerPC or the new Intel. Ever since I did this, my computer boots up to a screen with the loader bar and nothing loads. I did get a message after downloading something about a broken chain or pipe, can't remember. I have done everything you have told other people with similar problem except replacing the battery in back. Could that possibly be the problem?
Often the service being loaded tells you what portion of the system is having trouble. As the progress bar moves along the text around it should indicate what is loading. Try restarting your Mac a few times to see what it locks up on, and if it is the same thing each time. Then restart your Mac with the "Shift" key held down to get a safe boot. If the safe boot works and you have identified what the problem area is, then you should see if it is something you can reinstall or repair with Disk Utility.
Now if you can't even safe boot, then you will need to do an Archive and Install from the latest OS X install disks that you have. Archive and Install is selected within the install options section of the installer. The Options button is at the bottom of the window where you select the destination drive. That will fix your OS software without over writing your personal data. Then you should "Repair Disk Permissions" from the Disk Utility. At that point you can try to upgrade your system again.