Could be all of the above.
Do not attemt to restart.
Get expert help.
You could destroy the motherboard.
Rash, June 2010
same problem, need help
drednot, September 2007
Straight from the vaillant manual >F.28 means No gas;Insufficent gas ;incorect gas valve adjustment ; electrode defective ;ignition lead defect; Electronic ignitor defective,