Possibly due to the fact that I removed the ATMega328p from the board during an attempt to upload a sketch to a stand-alone ATMega328p on a breadboard. I put the original MCU back on the Uno and haven't been able to upload since.
The 'L' LED stays on constantly, even across a reset. When trying to upload a sketch, it says that the "programmer is not responding".
At this point, I feel inclined to blame the 16U2 chip.