I would suggest that your inability to achieve an erection for three days isn't really long enough to say you have a 'disorder'. Continuing to worry about the lack of an erection can itself be a cause, as can stress, or other aspects of your life, or lifestyle.
If you are still able to have an erection at night, or at other times, the issue is unlikely to be physical. However, if the situation continues, or you start to feel any discomfort or pain, it may be worth getting checked over by your doctor.