The default, self-signed certificate used by Apache 2 is called snakeoil, it’s generated based on the hostname (as reported by hostname -f) to files /etc/ssl/certs/ssl-cert-snakeoil.pem and /etc/ssl/private/ssl-cert-snakeoil.key when the ssl-cert package is installed. Here is how to regenerate the key and the certificate in case the hostname is changed:

# hostname -f
# make-ssl-cert generate-default-snakeoil --force-overwrite
# ls -l /etc/ssl/certs/ssl-cert-snakeoil.pem /etc/ssl/private/ssl-cert-snakeoil.key
... (check the last-modified-time)
# /etc/init.d/apache2 restart