You can sit there, tense and worried, freezing the creative energies, or you can start writing something. It doesn't matter what. In five or ten minutes, the imagination will heat, the tightness will fade, and a certain spirit and rhythm will take over.
Inspiration is wonderful when it happens, but the writer must develop an approach for the rest of the time... The wait is simply too long.
Any asshole can write a tone-row. It takes a composer to write a tune.
Even experimental composers, revolutionary composers, self-styled radicals are, in writing revolutionary music, recognizing the music that preceded them precisely by trying to avoid it.