avrdude: stk500_getsync(): not in sync: resp=0x00

Whenever I upload a program, this error message shows up: avrdude: stk500_getsync(): not in sync: resp=0x00

It was working just yesterday. but when I tried to upload a program today, it didn't work. I am a newbie to arduino, so please help!

I am using an arduino uno r3.

It would help to know which board you are using but here's a quick tip. Make sure the board selected in tools/board matches the board you are using. The board selected will be displayed at the bottom right corner of the arduino IDE.

Bill