I suspect the OP lost part of the latch assembly. If so, he's SOL since those small parts aren't sold separately.
The best bet is to bring the pedal and a couple of beers to a "dirty nails" bike shop or bike co-op, and see if the mechanic can cannibalize the part from the dead pedal box.
If the OP were to list his city of residence, someone might direct him the the most likely place to find that item.
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