Check inside the RAM access door to see if there is any RAM installed and make sure it's seated into their respective slots properly.
Make sure you are using just the Apple keyboard and mouse (or other supplied input device) and have nothing else connected to the IMac.
Try a PRAM/NVRAM reset.
Resetting NVRAM
- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
- Turn on your Mac.
- Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
- Hold these keys until the computer restarts and you hear the startup sound for a second time.
- Release the keys.
After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.
The iMac's internal watch-type PRAM battery maybe low OR dead and maybe need a replacement from Apple or Apple Authorised service/repair center.
Look at the bottom display stand and tell us all the info about this iMac that is listed there.
Is your Snow Leopard disc a gray colored disc that came with this IMac OR are you trying to use a gray colored disc from another Mac.
This will NOT work as gray colored discs from another Mac will never work because gray colored install discs are Mac machine specific.
You would need to use a white, retail copy of OS X Snow Leopard and, even with that, it would have to be the proper version of Snow Leopard.
The current purchase-able retail version of OS X 10.6 Snow Leopard is OS X 10.6.3. Some newer iMacs need newer versions of 10.6 like OS X 10.6.4, 10.6.5, 10.6.6 or 10.6.7.