Pages and Posts not showing after changing thems

I was using the old Codium theme and once I change to this theme – or actually even testing with any other – the pages and posts are not showing… the pages come up blank. They only seem to work with the old Codium theme. I have tried deactivating all plugins and that has not helped.

Thoughts? I need to get my sites Mobile Ready and the old Codium theme isn’t made for that but Codium Extend is.

Thanks so much!