Stk_getsync error

I just acquired a bunch of arduino boards from someone. While trying to upload a sketch onto the Arduino Uno via a USB cable, I get a sync error saying that my AVR dude programmer isn't responding. Does this mean that the board has been flashed with a different bootloader? If so, how do I restore it back to its original state?

The most common reason why this error occurs is the wrong selection of the Arduino Board and the Port arduino
connected to.

I had ensured the correct COM Port was selected (via tools -> ports) as well as the Arduino Uno board. The error still persists.

Do you have a clone or a genuine arduino? If you have bought a clone, chances are high that you might need to install drivers first.

No, its an original Arduino Uno board and is recognized in the Device Manage COM Ports as well. The guy I borrowed it from frequently uses an external programmer on his devices. I think he might have flashed a new bootloader onto it and I have no idea how to restore it back to its original state.