There are a multitude of possibilities. Has the immersion heater been switched off accidentally? Or not have been switched on, if you are new to the house. Has the fuse gone? Has the immersion heater itself failed? Has the thermostat failed? Are you sure the hot water is actually part of the Economy Seven set-up? It's not necessarily. Sometimes the immersion heater is actually in two parts - the lower part heats the whole tank on Economy Seven, the top part only works on "on-peak" and heats the top 6 inches or so if you need heat immediately and the rest of the tank has run cold, or is not on. Perhaps the system needs simply switching from the top heater to the bottom. You need to go through the possibilities logically, and you should find the problem!