First of all, make sure you are calling session_start()
before using $_SESSION
variable.
If you have disabled error reporting, try to turn in on and see the result.
ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);
The most common reasons that aren't mentioned in @dayuloli's answer:
Disk space problem. Make sure your disk space is not full, you need some space to store session files.
Session directory may not be writable. You can check it with is_writable(session_save_path())