Loss of interest in sex, or sexual problems such as under-arousal or premature ejaculation, can occur at any age and for any number of reasons. A general state of under arousal is often accompanied by other issues in life such as overwork, stress and/or relationship problems. Men are often fairly poor at recognizing or acknowledging psychological issues or they see it as a sign of weakness. Stress is often viewed as something that affects other people but many men simply aren't tuned in to reading the signs in themselves.
A lack of libido can sometimes be accompanied by erection problems. Rejecting lovemaking, or finding reasons to avoid sexual contact such as picking a fight or staying up to watch the late movie, can easily be interpreted by a partner as rejection. So, if your issue isn't with your partner it's important to open up to them and confront the possible reasons for the problem.
Checklist of Possible Reasons - in isolation or combination
If you think you fall into one or more of these areas then start talking the issues through with your partner and/or professional help. A loss of interest in sex is very often psychological but it can be physical or a combination of the two. If you think the issue is psychological then mood states can often be elevated by quite simple changes such as finding time to relax if you are doing too much, or, increasing your physical activity levels if your lifestyle is sedentary.