Super admin access to a forgotten WP instance?

Note: Super Admins only have meaning in WordPress Multisite. I’ve assumed below that you’re running Multisite, though your question isn’t tagged as such.

Super Admins are stored as a serialized array in the {$prefix}_sitemeta table in the site_admins record. In my local installation, with only the user ID adminpj as a Super Admin, here’s what I see:

mysql> SELECT meta_key, meta_value FROM wp_sitemeta WHERE meta_key='site_admins';
+-------------+--------------------------+
| meta_key    | meta_value               |
+-------------+--------------------------+
| site_admins | a:1:{i:0;s:7:"adminpj";} |
+-------------+--------------------------+

Unserializing the meta_value, I get:

Array (
    [0] => adminpj
)

So you can either a) see what your existing Super Admins are, or b) add your own username to the site_admins record.