I get a message saying 'Wintab service not available'. I looked up some forums and found this is a common problem with no real solution. The best solution I found is to go to SERVICES and restart 'TabletInputService' and 'TabletWacom', and it works again after restarting ZBrush.
Yeah ZBrush gets really upset when that heppens. Windows has been behaving itself the last few weeks, so fingers crossed that there won't be any more problems! If not, atleast the SERVICES restart solution only takes a minute :)
Ahh yes :\ Pixologic is right though that's not a ZBrush problem, occasionally the tablet service just fails to load and won't work in any app. ZBrush spits out errors while photoshop simply disables all tablet brush options as it assumes you dont have a tablet.