Roomba just would not advance. It goes for a second then it stops and move backward. If your Roomba shows this symptom, more than likely that its front wheel does not rotate freely. One reason of it is hair tangles the wheel as shown below.
You can either clean the wheel without taking it out with needle nose pliers. Or with a gentle squeeze, you can pull the front wheel assembly out and clean its shaft, which is much easier. Front wheel assembly is shown in the picture below. Roomba uses optical sensor to detect wheel rotation via detecting color change on the wheel.