Screen backlight working but no picture after digitizer replacement

I've replaced a cracked screen, all following the instructions perfectly.

However, when I've put it back together, it boots, back light turns on, but no image is on the screen - no apple logo or anything.

Once on, I can lock and unlock with the power button, and the home button works.

Any suggestions?

It doesn't seem to be the same problem as here: "Th screen stays black after a digitizer replacement"... because it's actually 'lighting up'.

I've done the usual hard reset with no luck