This is a problem I’ve had a few times and I haven’t been able to find any info on it online so hopefully it’s just me and easily fixable.
Basically whenever I’m working on a large project (for example my current one which is ~600mb), at the end of my session I save out a .zpr file. Usually I might save a few copies in case one corrupts or whatever.
Then when I go to load it back up, I open the project file… and nothing appears on screen. If I try to drag on the screen I’m just drawing the default 2.5d squares.
I have a bunch of tools available on the tools menu but none of them are the tool I was actually working on - in other words the tool that was active when I saved the project is just… gone.
The project file size hasn’t changed so I assume the data is still there but I just can’t see it.
Any help is appreciated. Currently using zBrush 4R2. Anything other info needed just say 
