I think the Pressure Relief Device (PRD) has blown.
It’s a safety device on the bottom of the heating tank.
It is fairly easy to replace however there is usually something that causes it to blow, often a kink in the hose or the showerhead is blocked, so check this before fitting the new PRD.
http://www.showerdoc.com/TRITO-82800450-Triton-Pressure-Relief-Device-82800450