Arduino nano doesn't accept the code

I have Arduino nano, maybe Chinese copy. I was working fine, but now it's not working. I can't upload any sketch on it. IDE display " avrdute: verification error, first mismatch at byte 0*0000 error " . Even I tried to burn bootloader again but failed.avrdute: verification error, first mismatch at byte 0*0000 error while burning bootloader is error. Please anyone can tell me which type of error is this . And how I can solve this.

Have you updated AVR boards to 1.6.21? If you have, select Tools > Processor > ATmega328P (Old Bootloader).