If Apache is installed as a Windows service, any errors will show up in the Windows System event log. To view the System event log, navigate to Windows Administrative Tools->Event Viewer and open the System log in the Windows Logs folder. Or, you can see the same errors by running httpd.exe at the command line. In my case, httpd.exe is located here: C:\Bitnami\wampstack-5.6.26-2\apache2\bin.
If there is a problem with starting Apache, most likely the problem can be fixed by editing the httpd.conf file located in the \apache2\conf folder. As already suggested, you can check the syntax of the httpd.conf file by running httpd.exe -t at the command line.
I found that the I had to
Change
SetEnv PATH "C:\Bitnami\wampstack-5.6.26-2\apache2\bin;${PATH}"
in httpd.conf
to
SetEnv PATH "C:\Bitnami\wampstack-5.6.26-2\apache2\bin;%PATH%"