My iMac won't boot. Every time, no matter what mode I try booting in, it boots and stays on a blank white screen
Try to boot up using Cmd + Option + P + R to reset the PRAM.

Also try booting up with Cmd + R for recovery mode.

It seems likely the main hard drive has failed.

If you can boot up into Cmd + option, you will be able to boot into a Boot CD or other recovery disc/USB drive to help restore your main hard drive.