To investigate and repair the situation you can activate the “superadmin” account.
On the file system edit the “xwiki.cfg” (location depends of the installation, but usually it should be found in the “WEB-INF” directory of the XWiki webapp). Look for a line like:
Uncomment that line and change the value “system” to something else (this is the password for the superadmin account). You need to restart XWiki to make the change effective.
Afterwards you should be able to log in as “superadmin” and the password you have choosen. Investigate the history of the “XWiki.XWikiPreferences” page, if there are any changes to the " XWiki.XWikiGlobalRights" which removed the admin right to the Admin user or the XWikiAdminGroup.
You will need to type in the URL manually as: https://my.wiki.server/xwiki/bin/view/XWiki/XWikiPreferences?viewer=history&showminor=true or something like that.
Of course you can first visit the wiki administration first to fix the rights of your admin user.
When you are done, do not forget to comment the “xwiki.superadmin” in the “xwiki.cfg” and restart the server a second time to avoid leaving the superadmin account active.
Some Documentation about that feature: