For the husband:  

Although you have been married for a long time, I would strongly recommend you to read books such as

• Ideal Muslim by Dr. Muhammad Ali Al-Haashimi
• How to win the heart of your wife
• Any other titles relating to this subject
• The prophet said "the most complete in Iman amongst you are those with good character. And the best amongst you are those who are good to their women" reported by Tirmidhi
• In another narration the prophet said "verily, a lot of women are circulating the family of Muhammad, complaining about their husbands. Those (husbands) are not the best amongst you"
• If you know that your wife has had a bad ordeal in the past, you should try your utmost in making her forget those memories by not speaking about them either directly or indirectly.
• Some women do change in their approach to marital relations after giving birth or from other past experiences so try to be understanding and help her by talking to her about them to give more confidence and relief.
• Show her that she is not all for bed purposes but rather as a companion and soul mate.
• Make her feel special and give her unconditional love, perhaps she has some feelings she is not disclosing to you so try hard in eradicating them.
• Your thoughts and temptations are not doing you any favours rather they are putting a hidden barrier between you both. You should know that whatever happened to her was a test for her from Allah and she managed to pull out from it. So, do not let them ruin your marriage. Also all evil thoughts are from satan, who is there waiting to split marriages. The most beloved action to him and the most hated thing to Allah is divorce.

For the wife: 

• Although you have been married for a long time, I would strongly recommend you to read books such as
• Ideal Muslimah by Dr. Muhammad Ali Al-Haashimi
• How to win the heart of your husband
• Any other titles relating to this subject
• Reported by Aisha that, "a woman came to ask the prophet about some matter, and when he had dealt with it, he asked her, ‘do you have a husband?' she said ‘yes'. He asked her,' how are you with him?' she said, 'I never fall short in my duties, except for that which is beyond me.' He said, ‘pay attention to how you treat him, for he is your paradise and your hell.'" Reported by Ahmed and Nisaa'ee
• The prophet said, "if a woman prays her 5 daily prayers, fasts her month (Ramadan), obeys her husband and guards her chastity, then it will be said to her; ‘enter paradise through whichever of its gates you wish'" reported by Ahmed and Tibraani
• Narrated by Abu Hurayrah that the prophet said, "if a man calls his wife to his bed and she does not come, and he goes to sleep angry with her, the angels will curse her until the morning." Agreed upon. This narration should be enough to make any woman pay heed to the severe warning by the prophet that the angels curses are upon those who do not respond to husbands sexual needs because the purpose of a woman is to fulfil that and for that they are made in order to produce offspring. We also know that Allah does not rejects supplication made by angels as they are His closest creation who are always praising and glorifying Him.
• Narrated by Abu Hurayrah that the prophet said, "By the One in Whose Hand is my soul, there is no man who calls his wife to his bed, and she refuses him, but the One Who is in heaven will be angry with her, until the husband is pleased with her once more" reported by Muslim
• The angles' curse will befall every woman who is rebellious and disobedient; this does not excludes those who are too slow and reluctant to respond to their husbands:

"Allah will curse those procrastinating women who, when their husbands call them to their beds, say ‘i will, i will.....' until he falls asleep." Reported by Tibraani

• Marriage in Islam is intended to protect the chastity of men and women alike, therefore it is the woman's duty to respond to her husband's requests for conjugal relations. She should not give silly excuses and try to avoid it.
• The prophet said "if a man calls his wife to his bed, let her respond, even if she is riding her camel (i-e very busy)." Reported by Al-Bazzar
• The prophet said "if a man calls his wife, then let her come, even if she is busy at the oven". Reported by Tirmidhi
• Narrated by Jaabir the prophet said "if anyone of you is attracted to a woman, let him to go his wife and have intercourse with her, for that will calm him down" reported by Muslim
• Narrated by Abdullah the prophet said, "there are three people whose prayers will not be accepted, neither their good works:
• A disobedient slave until he returns to his masters and puts his hand in theirs
• A woman whose husband is angry with her, until he is pleased with her again
• And the drunkard until he becomes sober" reported by Ibn Hibban
• The warning given to women whose husband is angry with her reaches such an extent that it would shake the conscience of every righteous wife who has faith in Allah and the last day. She is told that her prayer and good deeds will not be accepted, until her husband is pleased with her again.

Conclusion:

After this advice, if your wife should persist upon not responding to your sexual needs then you should divorce her. This is because a woman who can not have conjugal relationship should not be married in the first place.