Have to get a new trunk latch mechanism, I believe. If you have the auto trunk opener, sometimes when the motor dies the mechanism "sticks" in the up position, which won't allow for latching. Also, there is a lot of plastic in these and if the latch piece gets cracked, it can cause the same problem.
I had the same problem. my trunk was slammed toe hard and the metal bar that the latch latches on to got pushed down. if you connect a chain to it and pull up with force it should be put back into the right position and be able to latch
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