iMac display not working

I have an iMac 2010, 21,5" which has hardrive problem and replaced it. While connecting its LCD with display connector cable, the cable got damaged at one end. I got new display-port connector cable, but still display is not working.

When I connected with external monitor, its working fine.

I asked at a local repair store they told me that it was either the LCD panel, or the SATA/Board (Logic board?) which would even be more expensive.

I thought its a LCD problem, got LCD from ebay but still the display is not working.

I did a SMC/PVRAM Reset, but this did not solve the problem.

I used flashlight and cant see anything on the internal screen.

My question is whether it's possible to determine if it is the logic board, cable connection, the Backlight controller or the LCD-Panel.

Thanks for your answers,