I don't know how any of us could hope to give you advice that is as informed as, let alone better than, that of the person who actually did the mod and apparently has your unit in his possession.
There are any number of shots in the dark that any one of us could take. Is there a mute switch? Could it be somehow engaged or if that is not the case, has he checked the resistance between hot and ground of either the input side or the output side? Like Totem said, the MM/MC switch is a viable culprit, because both channels go through it. Could be a short, due to any number of other causes. Next I would check the gain devices in the MC circuit. If there is no short to ground, then the likelihood is that one of the gain devices bit the dust, but it would be odd to lose both channels at once that way. I bet it's something obvious.
There are any number of shots in the dark that any one of us could take. Is there a mute switch? Could it be somehow engaged or if that is not the case, has he checked the resistance between hot and ground of either the input side or the output side? Like Totem said, the MM/MC switch is a viable culprit, because both channels go through it. Could be a short, due to any number of other causes. Next I would check the gain devices in the MC circuit. If there is no short to ground, then the likelihood is that one of the gain devices bit the dust, but it would be odd to lose both channels at once that way. I bet it's something obvious.