Hello Skloss
That’s a fairly large file, and it will just depend upon your system specs as to whether you can open it at all. The file may have also become corrupted in some fashion, which can happen the closer the file size gets to your system limits.
By default, Zbrush doesn’t save undo history. You would have had to enable this for it to happen. Just remember that a Zbrush Project File saves ALL tools loaded into it, with all their subtools, and all those subtools have an individual undo history. So saving undos can rapidly bloat a large scene file, and I don’t recommend doing it casually.
I can’t promise you will ever be able to open that file again, but there are a couple things you can try.
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Open Zbrush in a fresh session, and open Lightbox ( , ) key by default. In the Lightbox Browser, find the Quicksave folder, and see if you can load any of the autosave file versions, if autosaving was enabled (it is by default).
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If the project file was created in the current version of Zbrush (older zprs may not work), try using the Tool > Load Tools from Project function.
If you can get any version of the file open, you want to save out your individual tools as tool files (Tool > Save as) immediately. Tool files save only the active tool, whereas a project file saves all tools, as well as quite a bit of scene information. It’s generally better to work on tools individually as much as possible if you will have a large, complex scene.
Read the following section of the documentation on Saving your work, and while you’re there, reading the entire “getting started” section may save you a few trips back to the forum.