A control valve has been inadvertantly turned off?
An air lock in the supply pipe is preventing flow?
The shower unit itself is defective?
I am unable to think of another cause - it probably needs diagnosis and repair by an attending professional (plumber).
Good luck - my profuse apologies in taking so long to make this completely free response. 08:05 BST 24 April 2009 UK
Peccavi, April 2009
