Why would an iPhone user suddenly be unable to text another i phone user?

My husband and I have both have iPhones (through Verizon if it matters) and for the last week anytime he's tried to text me it doesn't go through.

I had this before check you not sending it through imessege as it only works through wireless, click on the unsent messege and it should give you two options send by text or imessege. Hope this helps though sounds pretty confusing thanks

Problem with the carrier