It was 27-7 at the half, so the Cowboys were not scoreless in the 1st half like you claimed. And the 14 the offense gave away would still have been manageable with help from the defense.
Instead, the defense gave up an 8 minute, 65 yard opening drive for the Packer's 1st score. The Packer's 3rd score was on a 5+ minute 93 yard drive. You have to blame the defense for both of those
In short, the offense & defense shared equal blame for the bad 1st half.
As for the second half, the Cowboy defense gave up TD's on drives of 65 yards, 65 yards and 60 yards. The Cowboy defense gave up long drive after long drive after long drive.
In short, while the offense unquestionably got off too a bad start, the Cowboy defense never showed up at all.