'''Of course if it's in warranty, or, your eligible for exchange, this should your first choice option.'''
'''No Backlight, Video Present'''
Check and re-seat The "LCD Flex cable connection", making sure there is no corrosion on the leads (a "light" rub with an ink eraser {not sand paper!} can remove most oxidation). Also examine the header on the board it's self, if the "lock down" tab is broke, it's not able to push the flex cable up onto the headers pins, to make proper proper connection, inside the header (you can try, inserting a piece of nylon cable tie, in front of the cable entry point, to raise it up, to the top of the connector).
Still, the most likely cause for "No Back Light" is one of the serial string LCD that provide the light is open, or the 24 volt source is not present.
neither of these are really checkable, or repairable, so replacement is in order.
Last resort is to Swap the Logic Board into a "known Good phone" of the same model, to see if the symptom remains, or is corrected. If corrected, replace the LCD with a Known Good, preferable "New" LCD.
''' Backlight, No Video Present'''
You can try all the above, but, more than likely you're looking at internal logic board damage, and it's time to retire the unit, as there's not that many service organizations left willing to take the time, and, effort to do actual repairs on units this small.
Simply, We're getting older, they're getting smaller, and it's harder to see.
'''Flex Cable Note:'''
Be advised there are 2 LCD replacements available for the HTC PC36100 Supersonic / EVO 4G. The main difference is the width of the "flex cable".
The 2V6A is the "Small" cable, The 2WCA is the "Large" cable & they are not interchangeable. Also, the ID# for these parts, is obscured, by the phones inner plate (you generally cannot see it before removing it, and, the digitizer pad). The secret for choosing the correct LCD, is that, the size of the "small" cable is much smaller than the "Digitizer" connector (located just above, the LCD connectors board location).
charlie