I can never remember Vaillant models, but is the Turbomax the old one with the tall brass diverter valve behind the pump on the right hand side? If so you need to check the lever is operating the microswitch on the right hand side of the diverter.
If it isn't making contact with the switch, you need to order a diverter valve repair kit as the diaphragm is most likely perished.
If it is making contact, does the hot water demand LED light up on the control panel? If not, the microswitch may be knackered or the PCB.
If it does light up, does the boiler fire? If it doesn't, possibly a board fault. If it does fire, may be looking at a few different things.
If it is a newer Vaillant, they use an "Aqua Sensor" and a motorised diverter valve instead of a hydraulic set up, so you need to establish whether the boiler is recognising a demand or not, if it is then check whether the diverter is operating correctly or not.
On a simpler note, as Zeebeedee said, check whether they have no hot water at all or just warm water, the newer models have a pre heat function on the DHW, which may have been turned off when you fitted the timer, with this turned on, it gives hot water for about 10-15 seconds, it may have been turned on before hand and masking an existing fault with the DHW. To enable it, turn the hot water stat all the way down and then back to the desired temperature, an LED or a tap symbol should appear to confirm pre heat is on.
