Hi Michael,
I recommend contacting your hosting provider and restore your site to the date when it’s still displaying okay. Upgrading from X to Pro shouldn’t do it, there could be difference in header or footer, but the content and design will remain the same.
And for now, you can install your X theme downloaded from Themeforest and activate it from your site. Or, you can simply activate your X theme from your site, it’s probably still there as Pro installation doesn’t overwrite the existing X theme. Please do that first so we could see what’s the issue switching from X to Pro.
Thanks!