I'm not sure if you resolved this problem. But if you were the one to block your friend, then in your address book, there would be a section with an arrow entitled "Show Blocked Senders". If you click on this, it will reveal anyone you may have blocked and then it's as simply as clicking the "Unblock" by their name. If there isn't a section like this in your address book, then perhaps it was your friend who accidentally blocked you and they should check their address book. I hope this helps.