blank screen 27 inch thunderbolt

I have the exact same problem. When connected to my laptop, the laptop shows that the display is plugged in and working but there is no display, no back-light, nothing.

I am solving it by replacing the video controller main board inside the display itself. I know my particular problem is due to a bad ribbon cable connector on the main board. This particular ribbon cable carries all the video data to the LCD display.

If a replacment board isn’t available here:
[연계 제품이 누락 또는 비활성화 됨: IF359-000]
A replacement (re-manufactured) board can usually be found on eBay for between $50 to $200 depending on seller and location.

This guide goes over how to remove the main board. It’s actually a guide to replace the Mini Display Port/Magsafe Cable, but doing that requires removal of the main board so the guide still works.

Apple Thunderbolt Display Thunderbolt & MagSafe Cable Replacement