How to troubleshoot the USB or chip damages

Using Arduino IDE 1.x
1

My Arduiono Mega no longer uploads sketches. The Power led lights but co not want to read from USB. I get the followingt error.

Arduino: 1.8.3 Hourly Build 2017/05/31 06:33 (Windows 10), Board: "Arduino/Genuino Mega or Mega 2560, ATmega2560 (Mega 2560)"

Sketch uses 1460 bytes (0%) of program storage space. Maximum is 253952 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 8183 bytes for local variables. Maximum is 8192 bytes.
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer
An error occurred while uploading the sketch

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

Loopback test Connecting D0 to D1 returns nothing back from the serial monitor.

2

If loopback test (you did remember to keep reset grounded right? and you're selecting the right port, ie, the port goes away when you disconnect the arduino?) shows nothing coming back, the USB-serial adapter chip isn't working.

If it previously worked, but now doesn't work, I think your board is dead. Did you do anything that might have abused the power rails? (accidentally getting a voltage higher than 5v on the Vcc rail, or having Vcc connected directly to motors without clamping diodes seem to be known offenders here) The official boards (ones based on 16u2 as serial adapter) are particularly vulnerable to this. Replacing the 16u2 is not practical without hot air rework facilities and a lot of patience.