I remember reading somewhere about the ping diagnostic displaying in-accurate readings and not working properly. Also, having the add-on removed is highly unlikely (nigh impossible) to make your server dead (unless key blockland add-on which ping diagnostic is not).
Perhaps nobody wants to come to your server when you host. Try asking someone to actually agree to go to your server whilst you attempt to host. If it does not work, check your port forwarding and post here again.