I had the same problem with my Apache after installation. I checked all the resources and couldn't find the answer, but after checking the readme in the control panel, I found an uninstall for the Apache. I tried it, and it worked.
Here's what you do;
1. Go to your Apache folder in your xampp folder. Mine is located at C:\Program Files\xampp\apache.
2. Once inside your apache folder, open the file named "apache_uninstallservice.bat".
This file will uninstall apache service.
3. Then in the same apache folder, open the file named "apache_installservice.bat".
This file will restart your apache.
Now it should work, as this is how I fixed mine.
Maybe someone will add this fix to the questions and answers page of the site.