You need to take a gravity reading to see where you are at in relation to your FG. If it's stalled, you have a few options depending on the circumstances.
A couple of questions, and possible scenarios. Without knowing everything, I'm trying to cover all the bases here.
What temperature are you fermenting at? If it's in a cold area or has been, all you might need to do is warm it back up to 68-70 deg and see what happens.
If it's an actual stuck fermentation, you will need to pitch more yeast. What was your OG, what type of yeast did you use and did you make a starter? With a Belgian Dubbel if you are within style guidlelines OG would be between 1.062-1.075 and if you used a liquid yeast without starter, a stuck fermentation is a real possibility.
Let us know if any of these situations might apply.