Token request failed on Apps pages after we have changed our domain name

Hi
We have deployed our apps part in one of our office 365 public site and  it was working well.
But recently our client has changed their domain name (ie From https://xxxx-public.sharepoint.com to https://customDomain.com)
Site is working. But AppPart is throwing an error "Token request failed".

We are using the following code.
var token = TokenHelper.GetAppOnlyAccessToken(SHAREPOINT_PID, parentSite.Authority, null).AccessToken;
Can any one help us. Thanks in advance

S H A J A N