1. Firstly go and check your Apache Error Logs at: '/var/log/apache2/error.log'
2. There check if you are getting an error such as:
[Mon Sep 07 12:40:40.533934 2015] [core:alert] [pid 31413] [client 127.0.0.1:47560] /var/www/html/magento/.htaccess: Invalid command 'Header', perhaps misspelled or defined by a module not included in the server configuration
To resolve this open your Terminal by pressing CTRL+ALT+T and enter the following commands. Basically what you are doing is what is called the mod_headers:
sudo a2enmod headers
and then restart your Apache web server:
service apache2 restart
Now try accessing your website and it should work!