There is no need to modify the index.php for changing the setting of the home page. To change the home page setting you should have changed the setting from admin under the cms page menu option.
Now you have replaced the old index,php still your store is not working.
1) Check whether the file name is index.php only.
2) Clear your cache and check.
3) Also refresh the indexes of the website by going to admin->configuration->index management.
4) If admin is not accessible then manually delete the contents under the folder var/cache/