I would recommend a few things in this case:
- Check if there are any errors in your config files by running a config test:
If you have any config errors the above command would point you to the correct direction for further troubleshooting
- Check the status of your Apache:
systemctl status apache2
This could also give you some very useful information
- Check your Apache error log:
tail -100 /var/log/apache2/error.log
The above steps should provide you with all of the necessary information to get the problem fixed.
Hope that this helps!