Check if your website's SSL certificate has expired. Some SSL certificates need to be renewed manually, and you may not have done
SSL Server Test
Run an SSL Server Test to check the validity of a website’s SSL certificate.
Confirm Your Certificate Was Issued Well
A padlock icon displayed in a web browser also indicates that a website has a secure connection with an SSL certificate
Check Certificate Domain Name
If the certificate is only set up for one variation of your domain name, and you access the website using another variation, then it be a reason of this error
to know more about “Your Connection Is Not Private” Error