ZBrush won’t just stop running spontaneously. Something changed with Windows, which is affecting ZBrush.
The most likely candidate is the C++ redistributable. But let’s eliminate a couple other things first.
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Assuming that you’re running Windows Vista or 7, right-click on the ZBrush executable and choose Properties. Go to the Compatibility tab. Make sure that the ONLY thing that’s turned on there is the option to run as administrator.
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If that doesn’t resolve the issue, try uninstalling ZBrush and deleting its folder so that you’re back to a completely blank slate. Then reinstall ZBrush. If you no longer have the installer you’ll need to have Support refresh your download link.
If neither of those resolve the issue, then it’s almost definitely the C++ redistributable. When you reinstalled it, did you use the version that was in the ZBrush 4\Troubleshoot Help folder, or did you download from the internet? If you downloaded from the internet, the one that ZBrush needs is the 32-bit version since ZBrush is a 32-bit app.
The real catch is that Windows is often a bit dicey when it comes to uninstalling C++, which in turn can affect the reinstall. For some reason, Windows does not fully uninstall it. I have seen reports of people with your issue who were able to get around this by getting a more advanced uninstaller that forces complete uninstalls with no files at all left behind. You should be able to find such a thing with a Google search.