THe theme’s Customizations area should allow you to specify the header image (and even the background image).
Although this is not the place to get support for a theme (or plugin), look in Customizations first (in Admin area, under Themes) to specify your header image.
It may be that the actual image file was deleted. You could tell by using the Developer Tools in your browser to see all the requests for the site. If the image file is part of the page code, but shows a 404 response, then the image file is missing from your site. Go into Customizations and re-add the header image.