Hi Ru. 
Sorry to hear your change of domain host is causing access issues for your Discourse site.
As @pfaffman pointed out, if you are on our hosted service it shouldn’t be too difficult to fix. However, if you are hosted elsewhere (including self-hosting on a VPS), it should be a similar process to fix.
The first step is to determine where your Discourse is hosted and contact them. If you or the person who set up your Discourse is hosting with us you can message the @team group when logged into meta.discourse.org, or by emailing team@discourse.org. 