Site errors appeared overnite!

Can anyone tell me what these mean..and what I can do to fix them.
When I go to my site, the main page loads, but with this error on top of the page:
Warning: session.auto_start is enabled - please disable this php feature in php.ini and restart the web server.
When you click on any link you get this:
Fatal error: Unknown(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition <b>navigationhistory</b> of the object you are trying to operate on was loaded _before_ the session was started in /home/mshang33/public_html/catalog/includes/application_top.php on line 309

Does anyone have any info for me?

[michael_s]

Looks like your host updated php overnight on you and changed a few settings

I think you can put a flag in an .htaccess file like:

php_value session.auto_start 1

I am shooting from the hip on this one, but I think it should work.

Thanks!...I actually emailed my host and they turned off whatever it was they turned on!
Thank you for responding though!