I think i know what you mean... if you look on the +ve wire at the end that has come loose, there should be a male metal spade (albeit a small one). In the side of that, there is a very small flap of metal that sticks out to the side. If the wire gets caught or gets pulled back too hard, it can bend this flap the wrong way, disconnecting it from the rest of the block. Carefully bend it back so that it is at 45degress awat from the spade, and then it should clip back into the block. Be carefull not to snap the flap off though as it is fragile.
If that fails, your probably best off getting a new wireing loom from you HU manufacturer. They arent that expensive.
Hope that helps,
Alan