Why do online communities die? How can one bring them back to life once they have died?
All online communities are different and as a result they can fail in millions of unique ways. At the same time, the failure for all communities looks the same. A failing online community will have more users leaving the community than joining it. That is it. To stop the community bleeding, one needs to understand the reasons why users are leaving the community at that pace they do or why they are not acting at the rate that is needed to keep the community growing. When you have particular reasons, you can start looking for solutions and the most effective implementations (which are impossible to recommend without knowing the exact details).