I've been having this really odd problem lately and haven't seen anything like it before and am getting very frustrated. It might just be a simple fix, but I've been animating water flowing through and every time I save the file to render it later, it just ruins itself. I check the save multiple times and everything is fine but when I close blender and come back later it's just messed up and I don't know how to fix it. The domain box become visible like the pipe, it also become very small and the animation I had for the water is gone. I've tried also to re-bake everything but with no luck found. I would attach the blender file but honestly don't know how, so here's a screenshot instead.
You probably need to set up a proper location for the baked cache, within your project folder, preferably with a relative path, so that the fluid data is correctly saved in a safe place, and available next time you open your project file.
It may be getting deleted in the temp folder or unintentionally lost.
Use the path box visible in your screenshot under the Properties Window > Physics > Fluid > Cache path and point it to a safe location, possibly within your current project folder, or next to the blend file.
After that optionally you may manually move the cache files from the previous temp location if you do not want to rebake, or simply just bake again to the new folder.