The modern answer has often been “revival!” Luther’s answer to this question was: “catechism.” Carl Trueman writes:

Luther’s answer to the heart grown cold is well- established and catechetical: return to the basics of the faith, and remind yourself of those. it is also rooted in the idea that prayer is not simply a spontaneous, emotional response to God; rather, it is an essential, imperative part of the Christian’s walk. As a husband has no choice but to love his wife—it is a command, however he might feel about it at any given time—so the Christian has no choice but to pray, however cold his or her heart might be. And the best way to and tested approach of going back to the basics.