It comes from the minutia of life. A song on the radio might make me think of a character. Then, while cooking dinner or taking a shower the little spark of an idea grows and expands out into other characters, settings, plot ideas, etc.
Writer's block? Honestly, the only way to break through it is to sit down and force yourself to write. Do a few simply exercises without a specific goal in mind. It will come back.
Unless the block comes from burn out. Then I take a day or so off to do something else.