uploading error

dear all,

I am a newbie to arduino. I tried to uploading the code to my arduino UNO atmega328p with no problem in the beginning but now when i am uploading the code on it, then I had this message:

avrdude: verification error, first mismatch at byte 0x0000
0x0c != 0xff
avrdude: verification error; content mismatch

please help as early as possible this is final year project and i have my final viva on tuesday…

i have attached a screenshot of the error message…