I would ensure that your user is an ‘admin’ type. Then I would reinstall WP core files (via the update, if that is available; otherwise you will have to do a selective file transfer so as to not to overwrite your settings).
I would also rename the ‘plugins’ folder to ‘xxplugins’ temporarily, to ensure that a plugin is not involved.
And I seem to recall that if you rename the ‘themes’ folder, WP will default to an internal theme; not sure on that. You could also transfer the default (the ‘twenty…’ themes) themes via FTP, overwriting existing themes.
Also, check for a standard wp-config file, looking for any changes.