I removed five screws underneath, trim piece above the keyboard and pulled the keyboard away slightly with flex cables still attached. Now there's no video, but it boots up properly and plays Windows logon sound.

Anyone with knowledge of these? Seems like flex cable to me, but which one? Not my PC so all help is greatly appreciated.

Edit: alrighty; flex cable was indeed loose. Couldn't feel play at first but it regained backlight by applying keyboard pressure. Unscrewing and reseating solved the problem.