Ideas come and go, and sometimes they do not come back. We commonly refer to this as 'writer's block.' Often, the well has not run completely dry, but we are still thirsty for inspiration.
I find this phenomenon occurs to a certain extent when I finish a work of any appreciable length. I have poured myself out: I am exhausted. I must wait to renew my strength. Even if I want to write something, I lack the energy.
This is not a time to be lazy, but rather to rest. Instead, I indulge in the works of others: literature is fertile ground for the imagination. When I finish reading a story, and my enjoyment is high, I do not take in hand to write. Just like a good meal, a fine tale takes time to digest.
I will immerse myself in all manner of media: books, movies, television, poetry, etc. I rarely get ideas from these things directly. Rather, they give birth to my own creative notions. If I like a story, I have no desire to tell it again: I enjoy what they did.
During this time, my own ideas are growing and will soon bear fruit. This is not to say that I will come up with something new. In the thousands of years that people have been writing, it would be hubris to think that I could imagine something that has not already been entertained in the mind of another. The raw materials are the same, but it is how you prepare them, mix them, cook and serve them that provides the true culinary experience.
My ideas have been nurtured and will mature on their own. Often, I will take a stroll through my mental garden to seek out anything that is ripe for harvest. When the time is right, I will not need to tear the fruit from the vine: it will come off easily and fall into my hand. And then I smile, for I have some cooking to do...