First, you will realize that something is missing. You don't feel that 'fire' like you used to. You find you have doubts — a lot of them. You wonder if you ever really knew God was real or that Jesus is your Savior or if certain books of scripture are true.
There are many options you can choose as you realize you have lost your faith, but the best option; the only option that will lead you to your end goal of finding your faith is to reflect on your life. Reflect on the blessings you have and on the goodness you see because of the faith you have lived by.
If you can't see that because you have been slowly detaching for a long time, you should retrace your steps, meaning look back upon your past. Think of special times that you exhibited faith, made a commitment to God, felt the Holy Spirit, felt a prayer answered. Perhaps you will want to write those things down.
You may question if your special spiritual experiences of the past were a hoax. Try to remember that each person on this earth, including you, has a conscience, an inherent goodness, and the light of Christ within. If you felt peace and joy in those moments, it wasn't a lie.
Perhaps, though, you really struggle believing that what you once felt was really right. That is when you can talk to others. You can talk to friends and family who do have their faith, and who can testify to you. Most importantly, you can, and should, commune with your Heavenly Father. Pour out your heart to Him earnestly, sincerely, and humbly. Ask for His guidance, for His love, for His spirit to rest upon you and envelop you. Ask Him to help you remember, and to confirm to you the truth you once knew.
You may lose heart if you try these things, and nothing helps you find your faith. Be patient. Keep searching. Read your scriptures, continue to pray, fast, and talk to those who will help lead you back to your faith, rather than push you away from it. Running away from what you lost will not help you find it. In fact, it most certainly guarantees you never will.
This method will work if you are willing to hear God's answer. If you already have your mind made up, you will never be able to hear His voice.
In most cases, step six, replacing your lost faith with something else, will not be a necessity as long as you take the first five steps very seriously and sacredly. However, if you have done all of the other steps with full purpose of heart, and you still don't find your faith, perhaps God is telling you there is more out there for you. Just remember that it is in God you must trust if you wish to find and nourish your faith once more.
To all of you who are confused and unable to find your way, there is hope, and you can get the answers you seek.