Yes, it can go bad. MD 20/20 is an inexpensive fortified wine which barely last three days before becoming undrinkable.
After being open for a couple of days, a wine’s aromas will go flat, its native flavors will subdue, and the wine may appear overly sour or acidic.
Fortified wines like Port, Sherry, and Madeira have a much longer shelf life once they’ve been opened due to their high alcohol and/or high sugar content. MD 20/20 has an alcohol content that varies by flavor from 13% to 18%.
Without a doubt, the best way to extend the life of an opened bottle is to keep it in the refrigerator and to cork it with its original cork or with a tight-fitting bottle stopper.
This holds true for all wine types. For red wines, simply remove the wine from the refrigerator a few hours before you’d like to drink it to let it warm up a bit. A wine will deteriorate much more quickly if left on your countertop at room temperature.
To get the complete lowdown on how to deal with opened wine, read HuffingtonPost.com's A Guide to Opened Wine.