First put the httpd.conf file back to the defaults.
Then try again to use http://localhost
If it fails with an error message then read the \xampp\apache\logs\error.log file.
If Apache can't bind to port 80 then it usually means that another server is using the port - Skype and or IIS.
Find the culprit and if Skype then configure it to not use port 80/443 or disable it in the case of IIS.