550 Rename/move failure: Read-only file system

Hello. Recently I cannot edit a file in a remote folder through FTP. It says: 550 Rename/move failure: Read-only file system. How can I solve this problem? Hope that somebody can help me.
Thank You in advance.

ADDITIONAL INFO:
when I CHMOD the file, it returns:
550 Could not change perms on index.php: Read-only file system

It's a read only file system. It means you cannot change anything. Please contact your web hosting provider for assistance.

Regards,
Mat