Arduino Uno Not Uploading

Uploading sketches to my arduino uno worked fine. Now uploading won't work. I get the following error. Is there a way to reset my board? Using the built-in reset button does not help. Error:

Arduino: 1.0.6 (Mac OS X), Board: "Arduino Uno" Binary sketch size: 7,878 bytes (of a 32,256 byte maximum) avrdude: stk500_recv(): programmer is not responding
Please give any information on how to fix or reset the board.