TX and RX are always lit - No port is detected by device manager

I am writing this in hope for finding the solution.

I have arduino Mega 2560, it was all working by using it from external power of 12V. Next day, I again powered it up but it didn't work. I again tried to connect it to laptop to perform re-programming but all failed. error message is saying no port detected . in tool menu, port is greyed. in device manager, there is no sign of Arduino neither unknown or other devices. but whenever i just plug it in laptop, RX, TX and LED13 are always lit, when I press Reset button, LED13 flashes once, then happen nothing.

i replaced it with Arduino Uno R3, firstly it was working and I uploaded same program, inserted into my project, then again it is stuck. LED13 of UNO is always lit, no port is detected in laptop when i plug it in with laptop.

I am awfully stuck. I have to submit my project. Days are passing but my project is incomplete.. Your suggestion would be highly favorable.

Hmm, it looks like small ATmega16U2 for USB/serial not working properly at least. The question is how did this happen. I am recommending to check power at first on the regulator, chip etc., everything what you are able to check. Then if everything will seem to be ok, you probably will need to reload software in ATmega16U2. It is able to use UNO as ArduinoISP for this task.

I have the same problem too. Not on a mega but a Leonardo. The 4 lights are all lit and I could not detect the board on my USB.