IF PORT 80 IS NOT THE ISSUE!
Check to see if the port 80 is in use first as this can be an issue. You can do this by typing "netstat -an" into cmd. The look for 0.0.0.0:80 under Local Address, if you find this is in use then follow the solution from @Karthik. However, I had a similar issue but my port 80 was not in use. My XAMPP had wrong paths locations, steps to fix this:
1.Find out the Apache version you are using, you can find this by looking in Services (Control panel, Admin Tools, Services) and finding Apache in my case it was listed as Apache2.4
2.Close XAMPP.
3.Run cmd as admin.
4.execute 'sc delete "Apache2.4"' (put your version in place of mine and without the surrounding ' ', but with the " " around Apache).
5.execute 'sc delete "mySQL"', again remove the '' when you type it.
6.reopen XAMPP and try starting Apache
If you are having trouble with FileZill, Mercury, or Tomcat you could try it here too, but I have not tested that myself.
Hope this helps!