Of my website there's just the header online

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Another thing that might help is:

How to fix a corrupted WordPress installation

WordPress frequently releases updates, which you can install through WordPress’ admin interface. Usually, this works without a hitch. But with some (big) upgrades, the upgrade fails half way and leaves you with a broken website. Fortunately, you can still fix your website without any data loss, and this article tells you how to do that. Download a fresh copy of WordPress First of all, you will need to download a fresh copy of WordPress. You can download this from the official WordPress website…