Sorry for the delay in getting to your question.
Its very unlikely its the thermostat as they very rarely fail. Its usually either the heater itself or the control board.
What you need to do is remove the plastic control panel on the door and access the control board. Take a picture of the wiring and then disconnect them. Remove the board and then open the plastic casing and remove the board from there. Then have a look on there and see if you can see a burn mark or dry joint. If so then resolder it as this is the heater relay which does like to give dry joints. Then fingers crossed this will cure it and get it heating again.
But if that looks ok then you need to pull the dishwasher out and remove the right hand side panel to access the heater at the back. You need a multi meter to test this so test it and see what reading you get. It should be about 29 Ohms. If faulty then it will need to be replaced.
But if the heater is fine then its back to the control board in the door and you will need to replace this and that should sort the fault here.
Let me know what you find or if you need anymore info just get back to me.