If you are on econony 7 then your hot water tank will have two elements in it, one at the bottom and one at the top. The one at the bottom will be the one that heats water through the night and the one at the top is boost/manual. Either the thermostat has burnt out or the element is faulty in the bottom one. It is also possible that the timeclock/switch is faulty. An electrician should be able to test this and find the fault.