Have you worked on a render that came out really really good but now all you have is a posed figure without a background ?

What do you do when your creativity just shuts down on you ?

Walk to the park.  Go eat.  Bring music with.

Not that I make many renders, but for me the process can involve some rough sketches in the beginning to determine what is in the scene and where it is. I guess it's harder to provide context/background for a posed figure after the fact.

I sometimes go months without rendering anything and then sometimes I have so many ideas that I can't get them all done.

As previously mentioned, going for a walk in the park is a good idea - you might see something you want to work into a render.  Sometimes I go into a cafe and see a painting on the wall and decide to do my own version of it in Poser or Vue.  I was on a bus in Italy a couple of years ago and I saw a young girl sitting outside her house at the side of the road reading and it gave me an idea for a render.

If getting out and about is difficult, then take a look at online galleries.  I saw one of Margaret Keane's pictures online a while back.  I typed her name into Google and looked at her images - straight away the ideas come flooding.

I also once saw a scene of a city on one of the Star Trek films and thought - I'm going to do a version of that.

I would never try to copy an image, but looking at what others have done is good for inspiration, and sometimes it forces us to have to learn new techniques to get a desired result.

Start modeling.

But then again????

In the end I could not go out ...... I kept seeing everything in quads......... Pulling loops.... Sculpting real trees. . . .

Most fun was opening Blender;
load a figure mesh, any mesh, and sculpt a look alike.

I find movies and TV series quite inspirational at times.

Save it and let it rest for a while. When you come back to it, you will either know what to do with it, or decide "Gee... that's not as cool as I thought it was" and you'll delete it. Either way, your frustration level will drop, and that's a good thing.

Drink!