I am assuming that the unit is not totally dead and the only problem is no water.
You will need a meter.
Check across the water valve and the float switch combo for 120 volts when it should be filling.
If it is not betting voltage then either you have a bad wire from the timer to the combo or the timer is toast.
If there is 120 volts then check just across the float switch. If it has 120 volts across it then it is not closing and is probably toast.