Power issue, diagnostic lights switches off when connecting screen

I am a newbie at Mac repairs, and just attempted my first repair. The Imac was completely dead, so I ordered a new power supply and changed it, according to ifixit instructions. Still no life. After reading some tips, I attempted to reinstall my RAM cards, but stil no life. At this point, there was one diagnostic light lit. Tried again to change the PRAM battery, and got the RAM cards properly fixed in their slots. Upon connecting the Imac, three diagnostic lights lit up, and the fan was running. And then: When I hopefully re-connected the display, it was back to the one light, no fan and no life.

Any ideas?