A husband's secretive behavior isn't always negative. He could be up to something nice, such as planning a pleasant birthday surprise. In this case, don't probe and don't go looking for hidden gifts. You don't want to ruin the surprise.
On the other hand, your husband may be closed up about issues that you are curious about, such as his past or maybe a change in his present behavior. If you want to find out things about his past, open up a dialog about your past. Add in a little humor that will offer a light-hearted and comfortable atmosphere. This will encourage him to open up and join in the conversation. Don't use pressure to find out things he isn't ready to talk about. Forcing an issue doesn't offer a conducive atmosphere for someone to open up when they already feel apprehensive.
If your husband's behavior has changed recently and you suspect he's up to no good, observe and listen attentively. At least for now. You don't want to accuse him of anything that he isn't guilty of. This will cause undue conflict. If he's coming home late from work don't rip him apart. Maybe there are legitimate reasons for working late.
Watch to see if there are legitimate signs adding up that your husband might be betraying you. The guilty often tell on themselves little by little. This is why you listen and observe first and speak later.
If enough signs have accumulated, calmly say that you feel something is going on and have the need and right to know. Do not accuse or get hyped up, even though tempting. Allow your husband to talk and divulge what he's been up to. He may be truthful or he may be defensive and lie, but at least he will be made aware that you know something is off.
Remind your husband of how important he is to you and how the secrets are hurting your marriage. The way he reacts and the way you react to his reaction will have a lot to do with whether you can work through these issues. Offer an atmosphere that will allow him to talk. Your goal is to find out his secrets and bring him out of hiding so that the flame of your marriage can be ignited once again. If you close him off by attacking, accusing and yelling, you may never find out your husband's secrets.