It is possible that the wine has 'madeirised' ie got a bit too much oxygen, and this has resulted in a change in the taste, from the rich fruitiness that you expected. No harm if you enjoy it like that, but it can be frustrating if you don't ! The lesson is to try and keep the wine covered all the time, and make sure there is very little air when in the bottle. The other possibility is as Doris Pinks says, wine fly, although I would have expected it to taste unpleasant rather than like sherry.