if after pram and you system still wont turn on then the problem is the power supply on your imac is dead and need replacement. Please let me know if there is still any issue. Thank you.If you would like any additional information or have more questions please don't hesitate to ask!Please remember to only rate my answer when you are 100% satisfied. IF you feel the need to rate bad or poor service, please stop and click on the "Reply to Expert" link with the issue you have. I will be happy to continue further and do everything I can to provide you with the service you seek.Thank you,
thats definitely a hardware problem on the Imac internal power supply. Apple Store should replaced it for without any problem.you're welcome, please rate my service to you and have a wonderful evening.