iMac will not boot with anything connected via USB
Try holding down the Opt key (aka Alt) when you are booting. This should give the option of choosing a boot device. If you see anything in addition to the hard disk then it is probably because one of the USB devices has a mass-storage mode. This might make the Mac try and boot off that instead of the hard disk.

You can also go to Startup Disk in System Preferences to see if anything invalid has been selected by mistake.