If you have followed the steps detailed on this guide How To Enable HTTPS Support With SSL Certificates? you shouldn't have any problems get your certificate enabled.
What error message are you getting when trying to restart the apache service?
Try again with the following command:
sudo /opt/bitnami/ctlscript.sh restart apache
If no error is found, you will see a message similar to:
Also, check the following log files. You can find information about your issue there:
If you want to, share with us the apache configuration files that you modified and we will help you with them.