This error happens when the Mac cannot boot from its selcted start up disk. This can be caused by the installation being missing/damaged or your mac is set to boot from the wrong drive.
If you hold the alt/option key when you press the power button to turn your imac on, you should be given a choice of what disk you want to use as a start up disk. Select the the correct disk (usually Machintosh HD or simmilar)
I would suggest that you remove any usb drives before you do this to avoid confusion.