apache2 is not a native service
Solution 1:
Most of the time it happens because configuration file has syntax error. To see what is the problem run
journalctl | tail
Or
apache2ctl configtest
It will show you which line caused the problem.
If it doesn't have syntax problem, Remove and install it again:
sudo apt-get purge apache2
sudo apt-get remove apache2
sudo apt-get purge libapache2-mod-php*
sudo apt-get purge php*
sudo apt-get install apache2
sudo apt-get install php7.0 libapache2-mod-php7.0
sudo /etc/init.d/apache2 restart