Can no longer upload ANY sketches to Arduino Mega 2560 - timeout at upload

Hah, no way.

Why.
You want to know what is wrong with your system you have to do some tests.

You could have blown up your board due to some electronic abuse. That test sees if the communications is working between the computer and USB adapter on the board. If that works then you can test the bootloader and see if it needs reprogramming or whether you throw it in the trash and get another one.