Oh Goodness!  My wine is vinegar.  What happened?

1. Contamination of some sort.  Either your containers were contaminated or the room.  Vinegar is made by acetobacter which is a bacteria.
2. The sulfur level was non existent or too low.  Metabisulfite is an antibacterial agent!  The Romans knew that way back when. 
3. There was an air space on top of the wine for too long.  
4. Did you top your wine up with water?  This lowers the acid and alcohol. and could leave you with a good environment for vinegar. 
5. Bung knocked off or poorly sealed.
6. Dry airlock.
7. pH was too high and the acid too low.
8. Fruit fly invasion?  They carry contamination for vinegar. 

OK, so now what?  Why not just finish it as a vinegar?  Once it is vinegar, there is no saving it as a wine.  

If the sugar level was originally high enough to produce a full 12-15% alcohol wine, cut the wine-vinegar with 1 part of water (non-chlorinated) to 2 parts of wine.  You may add a vinegar mother if you feel you need it.  Place it in a really warm area of 90 degrees F for another 2-3 months.  When your vinegar is ready to use, rack off, pasteurize at 120 degrees F (stove top) and cool.  Bottle in clear or decorative bottles when you are ready.  The mother from your working jug may be used for another batch.  

You can make fruit vinegars using your wine-vinegar as a base by adding about equal parts of fruit (red raspberries, peach, apricot, etc), peeled if necessary, lightly mashed or chopped to the vinegar with 2 tablespoons of sugar or honey.  Cook in a double boiler (not aluminum nor cooper) for 10 minutes.  Store for 2-3 weeks, strain and filter if necessary.  Herbal vinegars can be hastened along by heating also.  For hot treatment of herbal vinegar, bruise or crush the herbs, strain out after 2-3 weeks, replacing with fresh sprigs for appearance, seal, label and gift.

Pick up a couple of books on vinegar, including the garden way Making and Using Flavored Vinegars.  Cider: Sweet and Hard has a nice section on Vinegar.