Using CiviCRM with Security Pro iThems plugin - page not found for contribution pages after SSL certificate loaded
Check the URL set in CIVICRM_UF_BASEURL in your civicrm.settings.php matches the updated location of your site.

Protocol may now be https:// instead of http:// - I'm not familiar with Security Pro plugin; it may need to be improved to account for CiviCRM URLs also.

There's no harm in making the entire site SSL, which keeps configuration simpler ... just redirect all HTTP traffic to the encrypted URL.