Mac Won't Boot after running Boot Camp Assistant

HI all. I could use your advice. My iMac no longer boots properly.

I ran the Boot Camp Assistant to partition my hard drive, and it successfully created the "BOOTCAMP" drive. When it asked for my installation CD, I tried loading a Windows 7 installation partition through a USB hard drive, but the assistant would not recognize it as an install disk. So, I closed out of the assistant early, leaving the Boot Camp partition in place but Windows non-installed. Eventually, I rebooted my machine in an attempt to boot directly from the USB drive and resume installation.

However, I found the machine will not boot at all. It powers on normally, makes the startup sound, but after a few seconds I'm left with just a flashing folder and question mark icon. Holding down Option-key at startup does nothing. Trying to boot from my Snow Leopard disc by holding down C or D during startup does nothing. Holding down Command-S does nothing. Holding down Shift-key does nothing. Resetting the PRAM does nothing. I have also tried booting from a Windows 7 install DVD -- the drive spends an extra 20 seconds or so scanning the disk, but then skips directly to the flashing question mark.

What can I do? Is there some way I can access the EFI to make sure the correct startup disk is selected? Why am I unable to boot from my CD drive?

Thanks for your help.