All relationships go through challenges. Some people seem to think that a "good" relationship is one that never has any problems and that it's always just happy and loving. The truth is that all relationship require dedication and work if you want to make it great. If you don't give yours the love, attention and work it needs it can become "ill".
One of the big challenges we all face in love relationships is that it can fade very gradually and usually we wake up at a stage where its either too late or where we don;t care enough to fix it. The first and most important step to heal a broken relationship is to decide whether the relationship is worth fixing. Staying together for the sake of your kids or for financial reasons will only destroy your own spirit. Its much better to be happy alone than miserable together.
So, if you decide that you want to heal the relationship then you will have to start building bridges. Regardless of what your partner did or did not do. You have to take the lead and start doing everything that you can do in your power right now.
Communication is the most important part. You need to lay your cards on the table - even if its hard to do. We tend to fall into communication voids as couples and certain things just never gets discussed. You need to break that and talk openly to your partner. Its also important that you are completely honest about your feelings towards the relationship. Often times when you "say it" things start to change. Your partners will also open up and you can start to make changes.
Negative ideas can fester in your mind. If you keep things bottled up it will start to poison you from the inside to the point where your health and well being becomes affected negatively. If there is a lot of negative feelings because of things that may have happened in the past then you need to forgive. Forgive and forget is a crucial step if you want to heal the relationship and move forward.
If the communication gap between you is too hard to bridge then you can look into couples counseling. Having a trained professional help you talk to each other can do wonders in helping both of you say what you really want to (and need to) say.