OK, so when I check the elements in your page, the left margin is set to 460 pixels.
Obviously that’s not where it’s suppose to be.
“Best practices” for modifying code is to back up the files on your harddrive before make changes.
I’m assuming you don’t have those. SO, worst case is that you take default or “clean” versions of header.php file and style.css file from original sources and copy them to your server. The downside is that you’ll loose any other changes or mods that you’ve already made.
BEFORE doing that, take the existing versions of these files and download them to your computer. That way, you can compare the known default “good” copies of the files to your “none working” ones now. The hope being you can find the code that has been changed and repair it back AND still be able to retain any other code you’ve previously modified.
Hope that all made sense.
Ron