LCD and USB won't function  

LCD and USB won't function

Hi, I've just assembled a MK3S kit and I'm having trouble with start-up. When I power on the LCD just shows rectangles. Read a few posts but nothing has worked - thought I'd check if anyone has any ideas. Things I've tried

• Rechecked each connection on board - including correct placement of LCD cables (if I switch them LCD is blank)
• Checked all fuses
• Disconnect all but LCD and power cables and try booting - same rectangles
• Update firmware via USB - just times out (COM port is there - if I disconnect cable it immediately fails - if connected it takes a few seconds to fail) - error below:

avrdude-slic3r -v -p atmega2560 -c wiring -P COM5 -b 115200 -D -U ...Using Port : COM5
Using Programmer : wiring
Overriding Baud Rate : 115200
avrdude-slic3r: stk500v2_recv(): timeout
avrdude-slic3r: stk500v2_getsync(): timeout communicating with programmer
avrdude-slic3r: Could not open port: COM5

Is there anything else I can try, or is it a dead board? The green light at the top of the board (next to USB port) is solid green when power is on, so assuming it's not a power/PSU issue.

Thanks!