Usually a failed HEATING ELEMENT or HEATER RELAY.
Disconnect power & remove the dishwasher from under the counter.
The HEATER RELAY is located on the right side of the D/W. A metal panel/shield (on the right side) must be removed to gain access for testing.
Testing the HEATING ELEMENT *can* be done from the bottom/front---but is easier if the D/W is placed on it's back---and removing the bottom cover/panel.
In many cases I have seen failed HEATING ELEMENTS or RELAYS on Samsung D/Ws---in homes which had an INSTANT WATER HEATER (AKA: Tankless Water Heater).
This type of Water Heater requires a minimum GALLON-PER-MINUTE of water flow---before it will *heat* the water.
Many dishwashers have a lower GPM flow rate than required---this results in NO HOT WATER at all during water fill.
The D/W heating sytem remains "on" for long periods of time---each time the D/W is used.
Eventually the HEATING ELEMENT or HEATER RELAY fails.
Part link for the heating element ==> Heating Element
Source: Samsung dishwasher DMR78AHS -- HE is displayed