macrogy Thanks for posting the screenshot
Can you add your admin and cPanel details in edit fields in order for us to take closer look.
For reference, You can follow thread post (5 point) procedure.
You may change the PHP version enabling us to troubleshoot the relative issue as mentioned however kindly ensure that you let the PHP version update stay for sometime when the issue is being investigated or, Can we try and change php version conveniently through cPanel option as the same would enable us to review without you having to wait until we troubleshoot with site being down for the certain duration
Also, Some troubleshooting steps you may try --
1. First and foremost, You may kindly update the template to latest version as suggested by teitbite ensuring to take backup of your site
2. Check if the MB string is enabled on PHP 7 (Ask with hosting provider)
2. Turn on error reporting to maximum to see for any specific error
3. Check with server log found in hosting panel for traces of error, if found
4. Try to disable 3rd party extensions one by one as there are some which are still not compatible with PHP 7