Blocked by CORS Policy

please need some help. My site has been running fine but today I suddenly started getting these errors and the site not loading properly and sometimes an Error 521 Web Server is Down. When I access the console from inspect tool I get this error message.

Access to XMLHttpRequest at ‘’ from origin ‘’ has been blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource.

Please can anyone help?