Did you try checking out that line of code in your functions.php file? PHP can be really unforgiving – perhaps it’s something simple that you can fix? When the PHP gods ain’t happy, nothing works. ??
Otherwise, try back tracking your steps. What did you do when you were adding that button into the header-right? Did you work on one .php file or a few? Comment out all the new changes and see if what happens. If you can see the site, then work from there.
Last resort – use an older version of your theme that was working – before your attempt to add the button – and reinstall it.
Good luck to you!