I derive no pleasure in my Salaat and Thikr.

Answer

The objective of ibaadat is not 'pleasure'. The objective is to gain the Pleasure of Allah Ta'ala by means of obedience. Continue with the Thikr regardless of the 'heart being closed'. Also, every night devote a few minutes in seclusion to meditate on death and the grave. Regardless of your feelings, do not abandon the acts of Ibaadat.