So just to be clear, does the unit boot to the OS, and you just don't see anything on the display or does it seem to get stuck in a boot loop where it keeps rebooting before the display is initialized? I'm guessing it's the former based on what you said, but they are two completely different scenarios, so want to be sure.
I'm having an issue with my imac again. About a year ago my Imac would restart itself and get stuck on a black screen and keep chiming. I took it to the genius bar and they installed a new power supply. After a day the Imac started rebooting itself again. I read somewhere online about some sort of start up file you can delete and it will automatically create a new one. After I did this my Imac was problem free for about a year and it is doing it again now. If I unplug the power cord for several minutes and plug it back in I can get the computer to boot up properly but it will eventually restart to black screen again. Does anyone know of this start up file and where I can find it in the computer. I forgot the steps to do it and I think it will fix the problem again. I'd rather not take it to the apple store because they couldn't fix it the last time and I paid for a new power supply that I didn't need. Thank you for the help.