If someone hasnt answered your messages for 3.weeks its either something serious has happened in their life, they are ghosting you or they lost your number or they are offended by something they heard or someone said about you. You know your them best. So you might have an instict on which it is. Can you reach them or know about them another way such as through social media or through a friend or family member? If this isnt an option. You could choose to do nothing and wait for him to contact you. But expect that.the situation might be that he has already moved on. Its not acceptable to not contact you for three weeks. As someone always has access to a phone of some kind, they can reach you through email facebook, snap chat twitter etc. If they say they cant. its lies. I think you already know deep down that you shouldnt be being treated this way.