Cron breaks after upgrading PHP
As PHP develops, some code that used to work in older versions fails in newer versions.  The CiviCRM code gets updated to accommodate those changes - but obviously if you don't upgrade you don't get those fixes.

Your fix is to downgrade the version of PHP, then upgrade CiviCRM, then upgrade PHP again.  You can find a table of supported PHP vs CiviCRM versions in the Administration Guide 

If you need help upgrading see Find an Expert  (including me!)