Replaced thermal paste.Now int. screen not working but all 4 LED on.

Hi guys i really need some help.

Few weeks ago i replaced the odd drive in my boss imac with and ssd kit from owc.

Everything was working ok but he said he recently had some problems with electric discharges so i took it apart again just to look if there is some problem.

With this occasion i said i want also to take the logic board out and clean it and add new thermal paste to the gpu and cpu.

Everything done..i put it back together but i do not have video on the internal screen..

I rechecked everything and realised i forgot to put the sync cable back.

I put it back but still no image.

So...i have backlight..but for sure no dim image or something like that..

The external monitor is working ok. And i have all the diagnostic led`s on and also chime is working and the system is booting ok and working with no heat issues or something like that.

What can i do???

What can be the problem???