Fatel error

I was working on my blog and must have mistakenly done something to a file, but I don’t know what I did, or how to fix it. When I go to my site all I get is:

Fatal error: Call to undefined function: add_filter() in /hsphere/local/home/mikeherm/4healthconcerns.com/wp-includes/default-filters.php on line 20

Anyone happen to be able to help me fix this?

Thanks,

Mike