Learn English – the word for raining without clouds

Perhaps you're referring to the idiom mind went blank.

Edit: I'm assuming you're referring to the sudden inability to recall information that you already know, in general. This could be someone's name (as in the example you provided), or it could be, say, the sudden inability to remember some factoid during an exam.

If I'm wrong about that assumption, however, there's this word:

lethonomia: 1. A tendency to forget names. 2. The inability to recall the right name.

A specialist in formal worship, i.e. the liturgy, is a liturgist. Understand, however, that this is a very broad term that is used differently in various traditions and denominations, and that the writing of prayers is only one of a liturgist's responsibilities.

In mainline Protestantism, a liturgist is typically a lay minister who helps to organize worship services. But in Catholicism and the high church, especially above the parochial level, a liturgist is a cleric whose role may include the composition of prayers as well as choosing hymns, existing prayers, and other variable aspects of a religious ceremony.

If a prayer is set to music, i.e. it is a hymn, the writer is a hymnist, or more archaically a hymnodist.