please confirm which model do you have 820 or 920.
I am assuming this is the one without display.
most likely cause is the flood switch at the base has actuated. This means water has leaked through the door gasket.
One way to check is to turn power off and draw the dishwasher out from under the sink. May have to disconnect the electric and water inlet.
Tilt the dishwasher to one side and wait for about 1 hour. Try the power on and see if the beep goes away.
You can access this switch by opening the chassis. It is item 5 on page 10 of this parts catalog
. You can use a hair dryer to dry the circuit board.
If no help then you have to call the technician as there is not much service information readily available for this model.
See this user manual
if you do not have one.