In order to reset the password for MediaWiki's admin, there are a few options:

1. The first and easiest one is to use the option 'Lost Password?' next to MediaWiki's login. In this case the password will be sent to the email bound to the admin user;

2. If the first option does not work for you, there is a another way to reset MediaWiki's admin password. Log in PhpMyAdmin or another MySql tool. Navigate to the MediaWiki database and choose the 'user' table. If you have set a db prefix, it would be mw_user for example. There find the admin user (by default WikiSysop) in the 'user' table and click the pencil button to edit it . In the 'pass' filed type the new password and in the 'Function' drop down menu select 'MD5'. This will change the password for the MediaWiki's admin.

If you have problems with the above, try contacting your web host for assistance. You can always count on Siteground's Support for similar problems.