Knowing the exact issue is important.
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5 apachectl configtest
journalctl -xe
systemctl status apache2.service
sudo nano /var/log/apache2/error.log
netstat -ltp | grep :80 # To know if a program is occupying port 80
FYI: Apache modules reside in following. Sometimes we have mismatch of module especially after switching php version. If the loaded version is different and available version is different, that causes error too.
1 /usr/lib/apache2/modules
Mac OSX apache not starting
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3 # source: https://apple.stackexchange.com/questions/79299/apache-on-osx-10-8-not-starting
sudo apachectl -T