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