Everything I've read on this subject indicates one of these: You have inadvertently put the number on your block list; the recipient has put you on theirs; or you provider is having an issue. Is it happening with all messages or just certain ones. If it's happening with all, it's most likely a provider issue. If it is happening with only one, check your blocked numbers to see if it is on your blocked list. Phone settings/blocked numbers. If it is not on your blocked numbers list, try calling the number. If the call does not go through, then the other party blocked you (either on purpose or in error). If you neither blocked nor are blocked, you have to contact your carrier to see if they are having issues, or somehow stopped your texting service.
