what is the cause of total numbness?

It is normal. Your sexual feeling is in your clitoris, not your vagina.

During intercourse your clitoris does not get enough stimulation. It is because of how you and your partner's anatomies work together during intercourse . So stick a hand or finger down there and rub during intercourse, or ask him to do it. Just like when you masturbate. Find a position that makes this easy. Show your partner how you masturbate so that he can learn how you like to be touched. Or take his finger and rub yourself with it so that he can learn that way.

And extend the time you have intercourse. Also change your position, angle, depth and tempo to grind your clitoris on him during intercourse. After 25 years you already know what makes you orgasm. Use that knowledge with your partner. The secret is in your own hands, so to speak.

Best of luck!