Well, i guess your body is trying to tell you that it needs a break.
Stop masturbating for a while and you will see that after some recovery time, your body will be as good as new!!
I´d try having a break from masturbation (several days) and looking for some sexually arousing situations (thoughts, movies, videos, meeting girls, sex talks, fantasizing, erotic stories etc...). If you get aroused, try not to masturbate immediately or try to stop before having an orgasm in order to accummulate the sexual tension a little bit. It should get back to normal after some time. Of course, there may be also other reasons such as stress, anxiety, physical or health issues, side effects of medication, physical or mental exhaustion etc...which may need to be addressed if they contribute to the problem.