iMac with Snow Leopard won’t start

When powering up my iMac the screen turned white with blinking folder with a "?" marked in the middle of the folder appearing on the screen (see image attached). Thereafter tried to boot up by inserting the Snow Leopard disk (which was purchased separately, the iMac came with Leopard when purchased). However once it has booted up I tried to use the Disk Utility but unfortunately my internal Hard Disk or my external Hard Disk (which I uses as the back up disk) were not detected. It only detected my optical drive and the Snow Leopard disk. Likewise when I tried to install the Snow Leopard it cannot detect any hd for the os to be installed on. Same result when trying to boot up using Leopard 10.5.2 (the original os that came with the machine). Any pointer to solve this problem ?