Dark Night of the Soul

In life, all Christians encounter dry periods when it seems they pray and pray, but God seems far away and unreachable. It seems God has gone on vacation or even abandoned us. This experience is more common than many people realize. I  have been there myself more than a few times in life. And because life can be very discouraging at these times, we all say things we later regret. These periods are what are sometimes called "dark nights of the soul" where  God seems like He is totally unconcerned and absent. There are times a lot  of us even curse God. Those are times when  we feel like we are at the end of our rope. It is at these times God  purposely "hides" Himself to build our faith. 
Job is probably the best known example of a time this happened to a believer. When Job experienced his trial and time of testing, his  wife told him to curse God and die. In other words, "Just give up, Job."
Job replied, "Though He slay me, yet will I trust Him." 
These are times  meant to build our faith to an extreme point. If we can trust God even when He doesn't seem to care or even exist, that is the beginning of the type of  faith which can move mountains. Unfortunately it is also a time which can  shipwreck our faith. We may fail these tests several times through our live before we finally break through. It's what Hal Lindsey calls "breaking the faith barrier".
Dark Night of the Soul or The Unpardonable Sin?
During these times, I personally have said things to God which would peel the paint off the walls. Yet I always come back after my anger subsides. It's because I know deep down that God is true even when I don't understand why He can seem so tough at times.
At these times, some Christians worry that perhaps they have lost their salvation. Or think that somehow, they have committed the unpardonable sin. One thing you absolutely cannot do is lose your salvation. If you were to  curse your biological father, would you cease to be his son? No, that is  something which cannot be changed. In I John, it says "we are born of God".  Literally it says we are "born out of God". In other words, God "gives  birth" to us- gives us eternal life which is the same life He has. It is life with a certain quality. Physical life has it's qualities and Eternal life has it's own special qualities. One attribute of it is that it has no beginning and no end. That is what the word "eternal" means. The eternal life you have, had no beginning. You simply can into possession of it when you trusted Christ.
Nor can you lose it. In Ephesians it says we are sealed by the Holy Spirit.  A seal signifies ownership and security. We are owned by the Holy Spirit  and secured by the Holy Spirit. It also says the Holy Spirit is the down  payment for our inheritance of Heaven. If you put a down payment on a house, but don't complete the purchase, the house owner keeps the down payment. You now have the Holy Spirit and you keep Him. If you could lose  your salvation and God were to send you to Hell, that means the Holy Spirit would be in Hell with you. Do you see how absurd that would be? It can't happen.
In regard to the blasphemy/unpardonable sin question, that is something a Christian cannot commit. The unpardonable sin of blasphemy against the Holy Spirit could only be one of two things. When Christ demonstrated who He was by His miracles, the Pharisees accused Him of doing the miracles through the power of the devil. Some think this is the unpardonable sin- ascribing the work  of Christ to Satan. Since Christ is not now here proving His deity through performing miracles, some think this sin cannot be committed today. Others see this unpardonable sin as rejecting the convicting work of the Holy Spirit. If this is the case, then that means only unbelievers can commit it. It is the same as rejecting Christ as your Savior. If a person does that, then they would be no more concerned about the unpardonable sin or any other sin, because the  Holy Spirit would not convict them anymore.
The bottom line is this. If you are an unbeliever but are still concerned about having committed the unpardonable sin of blasphemy, you haven't  committed it. If you are an unbeliever and no longer feel the convicting  work of the Holy Spirit, you may have committed it, or the Holy Spirit may just be waiting for a future time to convict you again of your need of a Savior. If you are a Christian you have not and cannot commit it. It is an  impossibility to do so, since you have already received Christ.
Many sins are considered blasphemy. Cursing God is blasphemy, but not all  blasphemy is what is called the unpardonable sin. Actually blasphemy  encompasses a number of sins such as disbelieving God's promises. We have  all committed blasphemy at one time or another even as Christians. If Christians could lose their salvation, then no one would get to Heaven. We  would all blow it at some point in our lives. Robert Louis Stevenson, who wrote Treasure Island, once said we all have secrets which would shame  Hell. That is so true.
The Dark Night When God Seems To Hide Himself
A baby who is learning to walk will sometimes reach out to his father and the father will back further away and reach out to the baby to force him to  learn to walk. The baby may get mad and cry because he thinks he can't reach his Dad. But the whole exercise is to force the baby to learn to walk toward the father. That is why God seems to hide Himself from us at times; to increase our faith in Him even when we do not "see" his working in our lives or sense His Presence.
I just started reading a book I got for my birthday about these dark nights of the soul. It's the second book in a series by Chuck and Nancy Missler called: "Faith in the Night Seasons- Understanding God's Will". You might give it a read. So far it is excellent on this question.
What If I Have Backslidden?
If you have confessed your backsliding to God, then forget about past failings. Now just do what the rest of us keep doing. Pick yourself up, dust yourself off, and start walking toward the Father again, whether you see Him or not. He is there even though He may be hiding in the shadows. It is all about forcing your faith to grow.