I have just made some changes with Html blocks on the home page of my site such as rounding corners and gradient with -web-kit and Microsoft.gradient type of codes and add some Jquery scripts. They all worked really well, however 5 minutes later, I am automatically signed out the page and now when I try to sign in again, the following warning appears on top of the login page and i cannot login.
Please help me about this since this is the website of the corporation where i am getting internship.

Warning: session_start() [function.session-start]: open(/tmp/sess_a1157967a3ae8847262f787863b73e4b, O_RDWR) failed: Permission denied (13) in /home/......./public_html/concrete/startup/session.php on line 18