Categories suddenly disappeared! Checked previous posts

This error appeared at the bottom of my site:

Warning: Unknown: open(/tmp/sess_1aebd114159cc6947e7859a21fd8d49a, O_RDWR) failed: Read-only file system (30) in Unknown on line 0

Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct () in Unknown on line 0

And all my categories disappeared even though I didn’t update a thing! I checked the other threads but couldn’t find the answer to my problem. The /tmp folder of my site doesn’t seem to be full (, and I don’t have root access anyway).

It seems like the same problem is happening all over, but I tried the possible solutions and nothing seemed to work.

Here’s my url: http://exsiren.com

Thanks,
Ariel