this may already be known but Im not sure so I will throw it out there. when doing a system restore the license can break. undoing the system restore may get it back
Yes. A system restore will do this. If that happens, ZBrush will give you an authorization request code. Simply submit a ticket at http://support.pixologic.com and we’ll get you taken care of.
If it’s during the weekend, you can make a post in the Registration Support forum instead. I check that forum periodically during the weekends to assist.