My Arduino Mega 2560 worked well untill yesterday. 
But Yesterday I connected a 12V Lipo battery to my arduino.After about 1 hour later, while i was controlling a servo,I heard a sound and now it doesnt work.
When I plug into USB, green light and orange light(L pin 13) turns on, But it doesnt show in device manager or arduino.When I press reset orange light turn off and turns on.
Sounds like you smoked it. Do you have 5V or 3.3V on the headers?
Thank you for reply.
But I cant understand what you say?
On arduino I have 5v and 3.3v headers And They give 5v and 3.3v without any problem.
I have measured it with multimeter
If you have 3.3V and 5V it sounds like the regulators are working.
But your USB/Serial interface chip (ATmega16U2) may be damaged. Does the Loopback test work?
Or the microcontroller (Atmega2560) may be damaged. Was your last sketch writing a High to D13? That would turn on the L LED.
You may still be able to use an FTDI Basic to download code to the 2560 even if the 16U2 is damaged. Do you have one? Or an Arduino?
Thank you for reply.
I reflashed atmega16u2 using USBasp.
But it doesnt solved the problem.
Sir, please can explain me what is Loopback test?
My last sketch was pwm code for servo attached on pin 11
I have also uploaded a led blinking code to atmega2560 using usbasp.(Pin 13 Led)
But it doesn't work.
Orange led(pin 13) stay turned on.
I have no FTDI or extra arduino
Thank you
loopback test doesnt work
Computer cant detect arduino serial port
Then you are out of luck. Time for another board. Be more careful next time.