Hi, Just to add my experience in the case it will help someone else.
My iMac 27 late 2013 packed up wouldn't boot error no disk present so I assumed the disk was bad. When I opened it i realised it had a Blade ssd, once stripped down and the logic board out. The post that secures the blade SSD to the board had broken off and the SDD was half unplugged. I re-soldered the post, refitted the SSD and reassembled.
On powering up I got the Chime but a blank screen. I followed the advice here and checked the display cables but no luck. I purchased a new cable but still no joy.
I opened it up again and I could see all 4 diagnostic LED's illuminated, so I assumed this meant communication to the lcd was good. From here I tried loads of things from the CMOS battery to resetting and booting to restore mode. Nothing made any difference.
Next thing I tried was to purchase a OWC envoy pro so I could test/recover data from the Blade SSD on another Mac. I could see the drive but it wouldn't let me mount it. after a few minutes I would disappear until unplugged and reinserted. it also seemed to get quite warm. So I bit the bullet and ordered an new Blade SSD.
In the mean time I removed the blade plugged in a SATA SSD and put it all back together. Finally it powers up and the screen is working! yay