If I have not seen my children in some time can I begin visiting them?
This will depend on the age of the children, the amount of time it has been since you saw the children and the circumstances which caused you to not be able to see the children. With younger children if there was some reason, for example drug use or imprisonment that caused you to be unable to see your children and you have reformed then you can usually receive some access to the children.
If your spouse will not agree to provide you with any access and you believe you have reformed then you can ask the court to provide this for you. The court will likely grant you only a small amount of access at first which will probably be supervised by your spouse or a 3rd party. The court may then grant increasing amounts of access after you have proven yourself able to care for the children and the children come to know you better.
With children over the age of twelve it will usually be up to the child whether they wish to restart or start their relationship with you. Many children will want to at least try to have a relationship with both parents but similar to with a younger child it is usually a mistake to try to force yourself upon them for too much time too quickly.