avrdude: verification error, first mismatch at byte 0x00d6 0xa4 != 0xa6

My code worked fine when I uploaded it onto my arduino a few days ago but when I tried today I got this error code:

avrdude: verification error, first mismatch at byte 0x00d6
0xa4 != 0xa6

avrdude: verification error; content mismatch

I also tried to install arduino on other computers but I receive the same error.

Please help!