Can save as image only once?

I’m very new so I don’t know anything about how Blender works.
Here’s the problem: can’t save an image. I know I should have clicked on the x beside the “render result”, but I just kept trying to solve the problem of saving that I can’t now bring it back. It’s just showing “object mode” (and others).

UV editor doesn’t work, the rendered image doesn’t appear there at all. It’s not textures; didn’t apply any, only material and some UV unwrapping. I thought it might be my graphic’s card, so I tried to save a cube we begin with as an image. UV editor didn’t show it either, but after I clicked on “render image”, the cube showed up fine.

So it seems to me like you can ONLY save your image immediately after you got your render result. Is that true?