Hello: It should take your heater only a few hours to bring the tank up to temperature from a cold start. So, there is a problem with power getting to the heater or there is a problem with the heater. There are troubleshooting instructions elsewhere on the site (see Tanklets or search under “electric no hot water”).
If you have a volt-ohm meter and are comfortable around 240 volts, you can troubleshoot. Otherwise, it’s best to find someone good with electricity 😉