A flashing question mark appears on startup
There is a folder with a question mark inside flashing when you boot your iMac
Wrong startup disk
Click the Apple in the upper left hand corner, select System Preferences, then click Startup Disk; select the hard drive that your operating system is located on, and reboot your iMac.
Operating system error
Try to boot from your OSX installation disk, by placing the disk in the disk drive. Next, reboot the computer while holding the 'C' key. Then, go to "Disk Utility" while the comupter is booted from the installation disk; choose "repair disk". If the problem is not resolved after competing these steps, back up your data and reinstall your operating system.
Bad hard drive
If the disk cannot be repaired and a new operating system will not install, then the hard drive is corrupt/damaged and needs to be replaced. Typically hard drives slowly fail, and may display signs of corruption before complete failure. Often times they will make a continuous "click" sound, or begin to make louder sounds as the problem gets worse. If your hard drive shows these signs, but is still functional, back up your files, and replace the hard drive immediately.
Not enough RAM
This typically isn't an issue unless you have upgraded the operating system on an older computer. First, downgrade your operating system so that the minimum memory requirements coincide with that in your iMac. You may also upgrade your external RAM, or upgrade your internal RAM so the new increased amount corresponds with the operating system you're trying to run.
Shutdown the computer, reboot while holding the Command, Option, 'P' and 'R' keys. Hold keys down until you hear the startup sound for the second time. Release the keys. If this does not work, it's possible your PRAM battery needs to be replaced.
CD/DVD Drive won't read
You put a disk into the drive of your iMac but it does not appear
No disk in drive
Open the disk drive and make sure you have a disk in the drive. If there is a disk in the drive, but it does not show up on the desktop, there might be an issue with the disk drive itself.
If the disk is a DVD, make sure your drive is one that can read DVD's and not CD's. Go to the Apple menu and click About This Mac. Then select More Info. Look for the Hardware category and then select the Disc Burning subcategory. Next, click on Disc Burning and it will indicate what type of drive in your computer.
The disk might be damaged or scratched. Insert another disk to see if it can be read, to determine if the problem is the disk itself.
Drive connection became loose
To check if the connection is loose, Open Applications then Utilities and Disk Utility, and see if the disk is listed in there.
Bad CD/DVD Drive
If none of the trouble shooting topics listed above solve your problem, it is likely that the drive has broken and needs replacement.
Unable to connect to wireless network
iMac can't find or connect to a wireless network
The network is down
First, you should try joining another wireless network. If you can connect, then the issue is with the previous network's connectivity and not the iMac itself.
iMac doesn't have an AirPort card
Make sure the computer has an Airport card, as it is an optional accessory at the time of purchase. To do this, click on the Apple logo in the upper left corner, select "About This Mac", and then click "More Info". On the left hand side of the System Profiler menu, find the category called "Network." If "AirPort" is listed within the options then there is one already in your computer, if not an "AirPort" may be purchased and installed if the iMac didn't come with one.
iMac has a bad AirPort card
If the card is there but cannot connect to any networks, then it may need to be replaced.