This is strange, because if file is renamed than a default K2 files should be used. Which means there is an issue with configuration or K2. Please send me joomla panel access. FTP access and a link to a page with example of this issue.
Joomla panel access You've send me is not working but I think I know what is wrong. Please go to K2 parameters and make sure using of k2 default css file is enabled.