I think you first need to verify whether a plugin or theme is causing the problem.
Temporarily rename the
wp-content/plugins folder to something else, then access the admin panel. If the problem does not reoccur, recreate the
wp-content/plugins folder, then move plugins’ folders in there in small groups to determine a suspect. You may need to activate those plugins.
If you can get to the admin page with plugins disabled, do a WP core update via the Updates page. Then start eliminating plugins.
If the problem is still there with
wp-content/plugins folder renamed, rename non-default themes (in
wp-content/themes) folders (the ones that aren’t ‘twenty-something’). That will cause your theme to default to the WP-supplied one. Get into the admin page and do a WP core update.
Then restore themes and activate them as needed until you find the culprit.
I suspect a plugin before a theme. And know that this will cause your site to be a bit ‘goofy’ because plugins are disabled. But that is what I would do.