If the Toshiba has a external output like most laptops, check to make sure indeed the video adapter is functional by connecting it to an external monitor. If that checks out, then the symptom seems to point to loss of backlighting. Your model is an LED backlit monitor so the circuit on the LCD would be handling that functionality. Either you got a DOA replacement panel and/or cable or worse, the driving circuit. I would try both cables. Otherwise the problem would seem to be the laptop motherboard panel driving circuit.