The Discovery link Tom provided on this thread is a great read. I copied some of the parts talking about the hatch below;

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We are more than a week late returning to port. We had problems relocating the capsule, problems getting the submersible to work. There was no time for even a cursory look around for Liberty Bell 7's blown hatch, the only real evidence of what happened 38 years ago. We also leave behind 87 unknown objects that turned up on sonar during our first sweeps for the lost capsule. Most likely, they will never be identified.

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Another mystery, the one most associated with this mission, also remains unsolved. The Liberty Bell 7 sank after its hatch blew open while it was floating in the Atlantic Ocean. Although some have speculated that Grissom blew the hatch accidentally, NASA officials believed his statement that the hatch ripped apart on its own.

Buckingham and the other members of the restoration crew haven't been able to answer that riddle, but they may have turned up another piece of evidence. Near the gaping hole where the hatch had been they've found a buckled strip of titanium. It's bowed a half-inch along that side, clearly the result of great force. But there are no signs of burn marks from the explosive device that supposedly blew the hatch open.
Buckingham suggests that perhaps the explosive cord never detonated, and instead the capsule may have slammed hatch-down into the ocean, buckling the titanium strip and popping the 70 explosive-rigged bolts on the convex hatch, one by one. That could explain why the hatch came off prematurely, he says.

But it's only a theory. Buckingham wants to examine photographs of the test hatches and results from tests on the hatch, before and after, and to consult metal experts.

Until then, the sinking of the Liberty Bell 7 will remain one of the NASA's greatest mysteries.