At first check if the apache server is running.
With Linux use for example
sudo ps aux | grep apache
With Windows and when Apache is running as service you can use
net start
at a command prompt to list the running services. When Apache has been started as command line application use the task manager. Note also that the Apache command line mode requires Administrator privileges with enabled UAC (since Vista).
When Apache is not running check the log files (
error_log at
/var/log/apache2 with Linux and
error.log with Windows. The default is the
logs directory of the Apache program folder).
When it is not running and there are no entries in the log files, check how Apache is started on your system and if that is executed. This depends again on the OS and the mode (Windows service or command line).