problem occured when upgrading to 2.5

I upgraded my blog to wordpress 2.5 using fantatisco interface. Everything went fine and once I check my site I found following error mesg on the top of the site.

Warning: Invalid argument supplied for foreach() in /home/mayuonli/public_html/blog/wp-includes/classes.php on line 92

Now I can’t browse to single page and can’t move away from the first chronicle mode page. Waiting for a technical advice
Mayu