Theme problem

I have recently moved my website to a new domain and host, the move went well however my pages were NOT loading eather saying ‘page not found’ or displaying a blank page. I started to troubleshoot by first disabling all plugins and it seemed non were at fault. I then changed the permalinks and put them back to see if that was the problem and it was not the case.

Now i’ve changed the theme to a default WP theme and the pages are now working. The problem now is that I have no idea what part of my theme will be stopping the pages loading.

Does anyone know the most likely cause?