We have a triton shower which did the same thing. Ours has a safety device to release pressure and for some reason it 'released' and let the water out as you describe. It's a one-use item - has to be replaced - but a new one was a lot cheaper than a new shower! Ring Triton for advice - 02476372222 - I found them very friendly and was able to order the part over the phone.
Graham, September 2008