You have 2 options: either contact the agency your used to ask why this is happening, or troubleshoot yourself.
To troubleshoot, first try and identify how the sitemap is being served, which will be either dynamically on the fly by a plugin (check the plugins list) or with a fixed sitemap_index.xml
file located in the root of the site on the server, at the same level as wp-config.php
, etc. (look for the file by using the file manager in the control panel (e.g. cPanel) provided by your host).
If there is a sitemap generating plugin installed (e.g. XML Sitemap Generator for Google), contact their support through the www.ads-software.com plugin support forum for help. If there is a fixed file in the root of the site, install the Health Check & Troubleshooting plugin, enter troubleshooting mode (which will disable all plugins and enable a WordPress theme for logged in users only) and test the sitemap URL. If it returns the sitemap page in XML format, then enable the plugins and theme one by one, testing each time, until you identify the conflicting extension. After that, check the settings of that plugin/theme, and or seek dedicated support from the plugin/theme support forum.
Good luck!