So I'm almost done with retrofitting an OD Grinder to CNC.  I'm using a C6 Speed Control Board and a VFD to control the speed of the headstock.  I had it working great and came in this morning and the spindle will not stay on.  I'll hit the spindle toggle and it comes on for about a half second and shuts off.  I've looked in all my configs and nothing has changed from the day before.  Any Suggestions??? 
--Chad

Figured it out.  There was a switch/input on the VFD that was in conflict with a brain that I am using for alarms/faults. 
Also, for others who may be having problems, I was putting a number in my Spindle just to be sure that it was going as fast as it could, S8000, and even with the brain disabled it didn't want to work properly, I.E. it still shut off as soon as it came on.   So I used S 550 and it worked.  Then I found out it was in the brain.