CiviCRM Search not working
Lots of folks are responding in the comments, but I'll try to give a non-technical answer.
Short Answer: You need to upgrade CiviCRM.
Longer Answer: CiviCRM is written in a programming language called PHP.  Over time, new versions of PHP come out.  CiviCRM must be updated in response.
It sounds like your webhost upgraded your PHP version to one that didn't exist when your version of CiviCRM was released.  So any issues introduced by the newer PHP are affecting your experience.  Newer versions of CiviCRM support newer versions of PHP, so you must upgrade CiviCRM.  If you can control your PHP version yourself (some webhosts allow this, others don't) you can try downgrading PHP as a temporary measure, but that's not particularly secure or sustainable.