From what I saw, it can be summed up as offensive line fail. Coming into this season, the Bears didn't have a one, but they had held up well in recent games against the Cowboys and Packers. However against the Giants, they completely fell apart and allowed 9 sacks in the first half, and knocked QB Jay Cutler out in the second quarter with a concussion. The offensive stats are ugly, but it really didn't matter how the rest of the offense played. If the offensive line is playing that bad, nothing is going to work.
Their defense is playing great and is certainly not the problem, although they get little pressure on the quarterback outside of DE Julius Peppers.
The Bears are still in the NFC North lead, but now the Packers are even with them in the loss column. The bigger problem for the Bears is how Cutler's concussion responds to tests over the next week because they certainly need him.