Answered by Sayyidi al-Ḥabīb `Umar bin Hafīẓ (may Allāh preserve him and benefit us by him).
I have not cried for a long time, not even in du`ā’. What should I do?
You have to have remorse for not crying from awe or yearning for Allāh. This regret is the first step. If this regret is true and strong then other steps will follow, like trying to make yourself cry. This should be followed by reflecting upon the sayings of Allāh and His Messenger, and the sayings of those who had fear of Allāh. All of these things are causes by which Allāh will return softness to the heart and return submission to Him.