There was a recent update in Google Sites which by default prevents sites from being . Anyone could iframe your site since you had no protection against it.
One side effect of this update is that users who have forwarded their sites to a domain will find that they will see a blank page when going to their domain.
You should remove the forwarding and then map it correctly. Follow the instructions in Steegle's FAQ - question 3:
I think it is best to map the site correctly but if that is too difficult and you want to try to continue forwarding your site do this: