DEAR SIR,
== ***Good backlight and "Toshiba" displayed on power up, then unresponsive MEANS YOUR MAIN MOTHER BOARD HAS A PROBLEM*** ==
***1.YOUR T CON BOARD, PSU BOARD IS OKAY AS TOSHIBA LABEL WAS ON DISPLAY IN STARTUP***
# GET AN USED OR NEW MOTHERBOARD OF model 65L5400U , REPLACE THE PROBLEM ONE AND START IT UP, IF DISPLAY OK THEN THE FIRST THING YOU DO CHECK THE WIFI SETTING, RESET AND DOWNLOAD THE SUITABLE FIRMWARE FOR THAT MODEL........AFTER THAT YOU GO TO TO RESET FACTORY AND RUN THE PROGRAM
# THE SOFTWARE WILL INSTALL ITSELF AND THEN LATER U MUST TRY YOUTUBE APP.....IF IT IS RUN WELL THEN YOUR TV SHOULD BE FINE....