10 Years After Having Brachytherapy For Stage One Prostate Cancer,

10 years after having brachytherapy for stage one prostate cancer, I can achieve an erection with 5 or 10mg of Levitra, but I can not ejaculate or experience an orgasm. This condition started about three weeks ago and I had very little trouble up to that point. I take 10mg of ramapril for blood pressure, but it has never affected my sexual performance in the past. I walk 4 or 5 miles per day and recently passed a strenuous treadmill test. What could be wrong?