You are misunderstanding the passage, so lets break it down so we can better understand it.
All Scripture is quoted from the King James Version.
Hebrews 6:4 through 6
For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
before we can break this down we have to go back to the verses before this:
Hebrews 6:1 and 2
Therefore leaving the principles of the doctrine of Christ, let us go on unto perfection; not laying again the foundation of repentance from dead works, and of faith toward God, Of the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment.
This is important because we are getting away from basic salvation and into an entirely different phase of Christian life.
Verses 4 through 6 are now talking about living the Christian life.
What Paul is saying here is what Jesus was referring to when he said:
Matthew 7:21 through 23
Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
So back to Hebrews 6:4 through 6;
For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come,
This is a description of having encountered the Holy Ghost, been saved, and have enjoyed the blessings of God knowing that they came from God.
If they shall fall away,
This seems to be where you have your misunderstanding: "to fall away" is not just missing something about being saved it is the opposite of that. In falling away you must take into consideration once enlightened which means you have fully realized your salvation and proclaimed Jesus as your savior, but then later deny that Jesus gives salvation
to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
Since Jesus has already saved you where is another sacrifice to salvation again?