Sometimes artists find themselves blocked when they let outside distractions get interfere with their art. Maybe they tell themselves that they will sit down and get to work "as soon as" some other task is completed. For instance, they may need to clean the house, pay the bills or complete a household project before they can work on art. Thinking about these things makes it more difficult to give art the attention it deserves.
Other artists may be blocked because they are spending too much time worrying about other artists. In her book "The Artist's Quest for Inspiration," Peggy Hadden mentions that artists may compare themselves to famous artists and convince themselves that they don't measure up. It's important that artists only worry about themselves, not the fame and success of others. Art is an external way to convey internal emotions and cannot be judged by how it compares to the expression of others.
Sometimes a looming deadline or the desire to finish a single piece can cripple the artistic process. The desire to finish a specific piece can stem from a need to get it to a buyer or simply because the artist wants to see it finished. This pressure it not uncommon and can be overcome when you clear your head or pushes through your doubt.
Once the cause behind artist's block is determined, you can work on getting back to work. Maybe you need to get away from the piece for a while, and going for a bike ride or to a movie might help. When you return to work, you'll be refreshed. Maybe you can try a different medium to get your artistic juices flowing. If you're mostly a painter, try sculpting or drawing for a while until your block disappears. Finally, sometimes simply sitting down and forcing yourself to get to work on the project can break artist's block. Put the paint to the canvas, stick your hands in the clay and sharpen up the pencil.