Since the need to restore a marriage relationship can be for many different reasons, we will look at underlying principles that the Bible sets forth for relationships in general and then marriage in particular.
The place to start is with the individual one-on-one relationship between a man or woman and the Lord Jesus Christ. As born-again believers, the success of any relationship with others is in direct correlation with the quality of our personal relationship with the Lord Jesus Christ. When we are out of fellowship with the Lord due to sin or mental attitudes that are contrary to divine viewpoint, we find that we are out of sorts with ourselves, first, and that spills over to our relationships with others. Therefore, restoring our relationship and fellowship with the Lord through agreeing with His viewpoint and resting in the forgiveness that is ours in Christ Jesus (1 John 1:9) is the place we must begin.
The above presupposes that one has a personal relationship with the Lord Jesus Christ through the new birth. That is being born again unto newness of life in accepting salvation through the gift of eternal life given to us in Christ. If that step has not been taken in an individual's life, then biblical principles are not the first issue to be addressed; one's eternal salvation or redemption is. This website has an excellent resource available to help to guide one to repentance and acceptance of the gift of life in Christ.
For the born-again believer, forgiveness is the position and privilege that we have in Christ, and because of that forgiveness we are commanded to forgive others. "Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you" (Ephesians 4:32). If we are believers, we are forgiven “in Christ,” and “in Christ" we also forgive others. No relationship can be restored without forgiveness. Forgiveness is a choice we make based upon the reality of our own forgiven state.
For the marriage relationship, the Bible has given us a very clear model that is opposite to the world's viewpoint. To restore a marriage relationship once forgiveness has been given and received, applying God's model will begin to bring the two separate parties into a God-honoring union. This requires a choice on the part of both parties. There is an old saying, "you cannot use what you do not know." Therefore, to learn God's model for marriage we must look into God's Word.
God ordained the first marriage in the Garden of Eden between Adam and Eve. When sin entered because of their disobedience, that perfect union was destroyed. Subsequently, God told Eve that Adam would be her "head" to rule over her (Genesis 3:16). (Compare 1 Corinthians 11:3; Ephesians 5:22; Titus 2:5; 1 Peter 3:5-6.) This "rule" has been overthrown by the modern liberal women's movement and has brought untold unhappiness to those who believe the "lie." There is also the human viewpoint that "all are equal." In a way, that is true. We all have equal access to salvation in Christ Jesus (Galatians 3:28). But to say that all in the world are equal in human opportunity, abilities or even power is naïve. God had a purpose for placing wives under the authority of their husbands. Because of sin, that rule has been both abused and chaffed under, and the result has brought chaos to the home and family. However, God does not let the husband off the hook. The husband is to "love his wife as he loves his own body" (Ephesians 5:28). In fact, the greater part of the responsibility of the marriage model is given to the husband. The woman is to obey her husband as unto the Lord; however, husbands are to love their wives even as Christ also loved the church and gave Himself for it" (Ephesians 5:25-29).
There is also a passage in 1 Corinthians 7 that lays down some principles and practical, personal, Spirit-led, advice about marriage from the Apostle Paul. This is all in accordance with the supposition that the individuals are born-again believers. This passage speaks about adultery, fornication, staying single and pure or—in order to avoid the pitfalls of passion and fornication—to marry.
God's marriage model works, but it takes commitment on the part of both parties to create a relationship with a balance of each individual's obedience to God and walking in fellowship with the Lord. It does not happen overnight. And, usually, if a marriage relationship has broken down, there are issues that need to be forgiven and put behind in order to move forward, and, again, that takes a choice and a commitment. Unwillingness on the part of either party will mean no restoration. The overriding issue is with each individual's responsibility before the Lord and then coming together before the Lord. Walking in forgiveness and fellowship would be a wonderful place to start to put the pieces back together again.