I have a similar problem - I've replaced my screen & digitiser and only the backlight is working, no picture. It makes the camera sound when I do a screenshot so I know the device itself is on.
The old screen was still working, but the glass was smashed hence the repair. As per online advice, I reconnected the old screen, and now I have the same trouble - the backlight is working but there is no image.
I figure this indicates I have done something to the inside. All the connectors seem to be connected properly, as I rechecked this during both assemblies.
My next step is to clean all the connectors with alcohol, and I will let you know if makes any difference, but my question is: is anybody able to please let me know the purpose of each of the two screen connectors? I'd like to know which one I need to pay particular attention to.
Here is an update:
Cleaning the connectors with isopropyl alcohol didn't help, so I figured it wasn't a case of the connectors being dirty.
I took it to a professional fixer and now it works perfectly. He said I had most likely accidentally damaged the LCD ribbons on both screens (or the one I received was faulty and I had damaged the original while removing). In any case, the message was that the ribbons are super, super sensitive. Super.
Based on this info, if you're going to proceed with the fix I recommend making sure your parts are from a quality seller, are sent in rigid packaging, and you are extremely careful with the ribbons when feeding through. If your old screen still shows an image, take the same care in removal as you do in assembly (so you have something to test against).
Though there is of course definite value in taking your phone to a pro, I'm disappointed I didn't get to see what was done to it (or get the satisfaction of a successful repair). Also I have to take his word that it was the ribbons and not some trade-secret extra twist to a screw or something.
The pro expressed criticism of internet tutorials but I didn't ask why. Perhaps it is that they don't provide sufficient, emphasised warning for dangers like these..(?)
If the occasion arises in the future I would definitely attempt another fix with this knowledge. I hope this is helpful in any amount and am interested to know how others resolve the problem themselves.