Hello and welcome. I am sorry for your current situation but no one knows what your ex is/was thinking. I know this is a tough situation because no one wants to feel replaced in general, let alone with his ex wife. I think what might be the toughest part of this for you is acceptance of the very thing you cannot change. I am not claiming this is a answer mind you but merely an observation.
Sometimes its more difficult to make sense of something that really doesn't. I live by the code "Actions Speak Louder than Words". His actions right now are saying he doesn't want or intend to communicate with you for whatever reason. The more you vent here, the sooner you might understand that you have no control over his actions despite 18 months of good times. I am sorry he is giving you the silent treatment but in time I hope you can start to mend your broken heart.