I recently had similar problem as OP. My MPC-2000XL's top row pad, all 4 of them, became unresponsive. When all one row or column of pads stop working, it's most likely that connection, not pad sensor itself, is the culprit, or I suspected that that is the case.
I opened my MPC and cleaned the sensor connector (the pad film part that connects to the mainpoard) but it wasn't very successful. I tried to insert the sensor connector the way that female connector on mainboard would contact a bit different area, because it seemed that conductive on the sensor film is a bit worn, making the first row of pads not sensitive.
To get the point, here's my solution that worked very well;
I inserted the nerf gun "bullet" under the film, so that now sensor's conductive would touch the unused area, making it better connection. Also, it gently pushes down sensor's conductive areas to each pins in the connectors on the board.
It works very well for me, at least now.