Old iMac won't start up and stuck in grey screen with spinning wheel

Hi, I have a very old imac which I purchased back in 2005, and has been working fine. I have left it in storage about 1 and 1/2 years ago.

Just now I try to restart it, the screen is forever stuck in the grey screen with the spinning wheel.

I tried many times off-on, hold down the apple function key but none worked.

I no longer have the rebooting CD with me. I wonder whether the computer can still be saved?

I have already backed up the contents so it doesn't matter if fixing the computer requires me to delete all my old files.

Thanks

If I am not wrong, I think it's tiger or earlier that it's using.