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

I don't understand what's going on with my ARDUINO UNO. I was using the microcontroller ATMEGA328P-PU outside the ARDUINO UNO board. Then during normal operations something happened. I don't know but the external hardware (an LCD connected to it) was not visualizing anything anymore. AT first I thought that some connections were wrong but then after checking many many times I gave up and hope that re-uploading the program would have solved something. Then I re-placed the microcontroller on the ARDUINO BOARD (All pins are not damaged at all and the micro is in the right position. The selected port is the right one and also the selected board). But I 'm not able anymore to upload anything. and if I press the button reset during the upload sequence (I searched first some solutions to this problem on the forum and Got this methodology) it returns me avrdude: stk500_getsync(): not in sync: resp=0x00. What's wrong? Help me please