For testing and learning purposes i have copied "header_required.php" from "/concrete/elements" to "/elements". Actually i did not even change anything, but now that i want to simply delete "header_required.php" in "/elements" i get an error message (Warning: include(/home/.../elements/header_required.php) [function.include]: failed to open stream: No such file or directory in...) and the whole site is broken by showing that message, even the dashboard is unusable, what means that the cache can't be cleared anymore.

How can i revert the system to the previous state?

Thank you,
Michael