Mould on top and bottom of white wine vinegar
It depends; if it's formed a solid disk and was/is floating on top, what you probably have is a vinegar mother, not mold.  It's a bacterial colony of acetobacter, and probably happened because your vinegar still had a little bit of alcohol left in it for the bacteria to feed on.

You have several options if this is the case:


You can leave it in the bottle; it will do you no harm.
You can filter it out with a coffee filter or similar and discard it.  Your vinegar will be fine.
You can put it in a bottle with (perhaps not so great) wine and/or the leftovers of bottles and make vinegar.