I think the pressure relief device 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-83301330-PRD-(Pressure-Relief-Device)-and-'O'-ring-(Pair)?ss=prd&pg=2