The best thing is to have a bootable backup of your iMac drive, such as one created by Carbon Copy Cloner or SuperDuper. You do have one of those, right?
A Mac can be started via a USB port, as a USB key or drive, but that device needs to have Mac OSX on it to boot, as well as other requirements. Best bet is to always backup your iMac hard drive with a bootable image created by those two tools mentioned above. For more info on usb boot, see:
If you don't have a bootable backup, or a way to get a Mac drive, things get very complicated.
First, try to boot your iMac in Recovery mode, holding down Cmd-r on restart. If that doesn't work, try for Internet Recovery, which is restart and hold down Cmd-Opt-R.
If neither of those work, find another Mac, connect it to the iMac with Firewire or Thunderbolt cable. Then reboot the iMac holding down the 't' key, which puts it in Target Disk Mode. If this works, the iMac hard drive will appear as a mounted drive on the other Mac.
If none of those work, you are going to need to take the drive out of the iMac, so be sure to read thru the iFixit guides.