Imac Help

When I turn on my imac, it doesn't open... It switches back and forth between telling me to turn on the mouse and the keyboard. The keyboard is plugged in through USB connection and the mouse is wireless with brand new batteries. Wondering if there is a command to use on the keyboard when turning on the computer to do a hard reset... I have nothing actually saved on the computer so resetting it wouldn't effect me. Help!! TIA :)