This error showed up when I went to my site!

Hi everyone!

I went to my company's site, and when I got there this error showed up:

Fatal error: Cannot use object of type WP_Object_Cache as array in /home/leetlink/public_html/wp-includes/cache.php on line 450

How do I fix this error and get my site back? All I did was install anti-spam plugin yesterday before I left the site (and it was still working then).

Thanks for the help in advance,
Nathan P.