trouble with posting

hello,

I was just about to post my first blog post. and its weird cause i had posted a trial post a few weeks ago..but now it says this:

Fatal error: Call to undefined method wpdb::get_blog_prefix() in D:\Hosting\5765220\html\wp-admin\includes\user.php on line 260

and in the USERS section of my dsahboard it says this :

Fatal error: Call to undefined method wpdb::get_blog_prefix() in D:\Hosting\5765220\html\wp-includes\user.php on line 445

everything else seems to be working fine though….please help! i’d be so grateful!

my blog is http://amaturecouture.com