Fatal error

Help!

I was clicked the automatic upgrade from 2.9.2 to 3.0 and this error came up:

Fatal error: Call to undefined function is_multisite() in /home/content/d/t/h/dthomsen1/html/wp-includes/wp-db.php on line 505

I can’t log back in does anyone know how to fix this?

Thanks for your help,

Dan