I have the same problem. Only white screen appears, the blue+green led is lit near USB port. Also tried everything. Measured serial TX & RX with oscilloscope, RX is snapped to 3,3V, but TX is stays at 0V. Can't see any hw failure on PCB, caps are in good condition (visual inspection only). Current reading is ~130mA @ 24V.
Please help, how can i save my board? (Warranty is off by now)