I have an Arduino Duemilanove and an Arduino Mega 2560. Up until today, both have always worked fine. I've been working on a project over the past few days, but when I plugged in the Mega today, it just would not run the sketch I had been working on.
The sketch uploads (pin 13, RX and TX LED's flash in the usual pattern), and the Arduino IDE says "Done Uploading" with no errors. I went back and tried the Blink.ino example sketch, and even that doesn't work. As above, the upload seems to go fine, based on the LED's and message in the IDE. If I manually reset the Arduino after uploading by pressing the reset button, I get the usual two blinks on pin 13, then nothing after that. As far as I can tell, the sketch just will not run. I also tried the BlinkWithoutDelay example, and got the same results.
To check that the IDE and my computer were fine, I uploaded the Blink sketch to my Duemilanove, and it runs fine there - pin 13 is blinking away happily on the Duemilanove. I also verified the serial connection to the Mega with the loop-back test from http://arduino.cc/forum/index.php/topic,73748.0.html
Something I noticed, is that if I open the Serial Monitor from the IDE when the Arduino Mega is plugged in, pin 13 lights up and stays lit until I close the Serial Monitor. Odd, since it should be blinking according to the Blink.ino sketch, whether the Serial Monitor is open or not. Not sure what this means.
-Windows 7 64-bit
-Arduino 1.0 IDE
-Arduino Mega 2560 R2
-Yes, I have the proper board ("Arduino Mega 2560 or Mega ADK") and serial port selected in the Tools menu.
-No external circuitry or wires connected aside from the USB cable
Anybody have an idea of what is causing this issue? I am inclined to believe that the bootloader is OK, because the upload appears to be working. I would say that I'm fairly experienced with Arduino, but this is something I've never seen before.