I am 23 and cannot maintain an erection. i did not have intercourse in 2 years but with my girlfriend i cannot get erection. what can be wrong? treat?

It depends: In some cases this can be anxiety-based, whereas in others it might be physiological. A good way to tell the difference is if you are able to get an erection while watching pornography or masturbating. If so, there's a good chance it's anxiety-based. Treatment in this case would be with psychotherapy, whereas physiological would call for medicine. They're not mutually exclusive, of course.