This is known as "seizing". The most common reasons for chocolate seizing as you melt it are overheating and the accidental introduction of water (for instance if your bowl was a little damp when you put the chocolate in, or if you were using a water bath to heat your chocolate and a drop of steam found its way into the bowl). Unfortunately chocolate that has seized cannot be salvaged and you will need to start over with a new batch. It's very important to melt it slowly, and ensure your bowl and stirring utensil are fully dry. If your chocolate seizes, you can stir in a little oil or chocolate and make a chocolate sauce, but that's about the only thing you can do with it.