Q: Is there one thing that can completely block my prayers?
A: Absolutely. It's a lack of forgiveness. You can pray any prayer you like and it won't do you any good until you forgive other people their sins. Prayer without forgiveness is like a car without an engine. Or a body without a soul. It simply doesn't work. Jesus Christ said after He taught the principles of prayer, "For if you forgive others for their transgressions, your heavenly Father will also forgive you. But if you do not forgive others, then your Father will not forgive your transgressions." (Matt. 6:14-15)
Jesus also told a story about this, and I think it's one of the most intriguing parables He ever told. It's the parable of a guy who's been forgiven effectively a $20 million debt, a debt he could never repay if he lived a thousand years. So he gets forgiven; then he runs out and finds a guy who owes him what amounts to a 20-dollar bill and he says: "Pay up." And when the man can't pay up, he hauls him off to debtor's prison. Well, when the Master found out about this, He was upset. You can think what a horrendous thing for us humans to do, when we've been forgiven what amounts to an unpayble debt, to then not forgive others whose debts by comparison are like a 20-dollar bill. So we need to forgive people — that's one of the principles that Jesus Christ taught when He taught us the principles of prayer.