Updated and now my site has error and won't pull up

When i logged into our site to do a little work it said i have an update, so I clicked on it, and now this is what I get: Fatal error: Cannot redeclare sticky_class() (previously declared in D:\Hosting\5635595\html\wp-includes\post-template.php:584) in D:\Hosting\5635595\html\wp-includes\deprecated.php on line 3192

I cant even login to fix anything. We had someone set all this up several years ago, and he just showed me how to login and update just the information, so I’m unsure of how to do anything else ??
The way i would login is, our site address with /admin after it, and then put in my username and password..but now cant get to that.
Please help!! I have to get this site back online.
Thank you!