I don’t like using files from the image library in my designs, just for this reason. Too easy for some random user with admin access to delete the logo or the header and screw up the whole site! But it does make it easy to change.
Some questions and answers:
Do you have a copy of the header? There is no restore feature that I know of. If you have a nice host they may have a backup of the site that we could dig the file out of. Contact your host, ask if they can send you a zipped backup of your site. Or you can make a new header image fairly simply…
You can fix this several ways, the easiest would be to just re-upload the file. Then you can tell WP to look at the new file, under Appearance->Customize or perhaps under Appearance->Theme Options, it depends on your theme.
Or you can do it the permanent way, by editing some code in your header.php and pointing to the new file directly. Otherwise you need to find the interface for designating the header in the admin.
Can you give us a link to your site so we can take a look?
