Yeah, a few...welcome patbj, by the way. you're going to have to reset the smc/pmu then see if a pram zap will fix anything. the first one is easy. You shut down your iMac, if it isn't shut down already, unplug it, wait about a minute, hold down the power button for about 45 seconds, then take your finger off of the power button, plug it in again, and press the power button and see if she starts up. you might also want to make sure the outlet into which the iMac is plugged is good, say, by plugging a lamp into it. if it does, hold down the Apple (cloverleaf), Option, P and R keys all together for 3 chimes, then let go of them and let the iMac start up as per usual. You should have a usb keyboard plugged in to do this, as Bluetooth keyboards can be pretty tricky and finicky for starting up your iMac.
hope that helped you a bit
john b