A blank display is almost always a broken nipple or crack in the VFD so that it is no longer a vacuum. Examine the VFD carefully - look for any signs of damage and that the nipple is in tact. The slightest crack is enough to reduce the VFD to scrap glass.
Another possibility would be catastrophic failure of the capacitors near the VFD. Check to see if your VFD is on, but just very dim. There is a
FAQ article about the capacitors around the VFD, and how they can go, leaving you with something unreadable in daylight.
Also check the connector going from the display board to the main board. If the buttons and knob work normally, though, this problem is highly unlikely.
Lastly, do you see any display at all when you boot?