The “headers already sent” message often means that there is code that is trying to write content (like an echo() call, for example) after the server has already responded with the main payload of the page. If you have written code for this site yourself, I would look for something like that that you may have changed recently.
If you have not written any code, and are more of a site administrator, it’s possible that this problem might be coming from a plugin that you recently activated or updated. If you think that might be the case, and you have access to the database, you might try deactivating the plugin through the database. Here is a decent article on how to do so: https://perishablepress.com/quickly-disable-or-enable-all-wordpress-plugins-via-the-database/
Good luck!