After installing RAM into my 27″ iMac the display was only showing static. Several trouble shooting attempts and a visit to BestBuy later I was left with the answer that my logic board was dead.
As a last resort effort though I plugged the iMac into my UPS at home and after a few seconds of black screen anxiety it booted! Turns out iMac’s are a very picky machine when it comes to power supply so be careful plugging directly into a wall or cheap surge protector!