Did you still need help with this issue?
If so, lets start by removing any USB sticks, SD cards, remove anything connected to the USB ports on your imac except your Keyboard , and mouse, and monitor.
Disconnect EVERYTHING besides the keyboard mouse and monitor.
T hen, shut down the machine, unplug it from power.. Then hold the power button for 20 seconds.
Then, plug it back in and try this repair first
Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
Turn on your Mac.
Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
Hold these keys until the computer restarts and you hear the startup sound for a second time.
Release the keys.
Then, see if the mac starts.. If it does, let me know as we have further repairs to do.
If it wont start, let me know if it shows any screens or errors and get back to me so we can carry out further steps as needed.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.
Senior Technician and Network Switching Systems Operator / Maintainer.