It could be a number of things. First check permissions level using a FTP manager. It could also be a defect in the .htaccess file.
There are a lot of hits for the problem on Google:
550 can't change directory to /public_html: permission denied - Google Search
http://tinyurl.com/zu3wf
Checked the permissions which have been changed to a value of 0. cant get into the htaaccess file since the permissions have been changed and god knows what else.