What kind of stimulation do you have in your sex life ? Remember the brain is the biggest sex organ in the body.
What kind of fantasy thrills you. Just keep dreaming up stuff, till something triggers your erection. It doesn't have to be something you'll act out in real life. For example an orgy. Multiple nude people doing crazy things.
Even semi erect men are able to climax. Keep pretending till you hit on a theme that hardens you. Thank you for your question. Keep posting your progress. Maxy