I have 6 Atmega328-PU's that stopped flashing after I uploaded a program that causes it to endlessly watchdog. The chips work, I see the expected output of the program on the serial console. I tried burning the bootloader again(the process succeeded on all of them), but I still cannot flash.
The infamous error when trying to upload new programs:
avrdude: stk500_getsync() attempt 1 of 10: not in sync: resp=0x00
avrdude: Send: 0  
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding
I have three different genuine Duemilanove boards I've tested as well as three different computers. I have other micros that never ran the code I can till flash perfectly on the boards.
I just tried burning the bootloader on each chip via https://www.arduino.cc/en/Tutorial/ArduinoISP. That succeeded, and I was able to upload a sketch to each micro now and verify they work! Thanks! What could I have possibly done that would require the bootloader to be burned again?