my arduino uno was just working fine now I get errors

my compiler and microcontroler has been working fine all day and just now im getting the same error. I hit upload and the progress bar will make it all the way to the end and all the sudden it pauses for a long time and then i get this message avrdude: stk500_getsync(): not in sync: resp=0x00 what is this and how do i fix it?

please help thank you