A few people have done upgrades on their WHM/CPanel found the default change to 'Cookie IP validation' is now giving them the message that their IP address has changed. The easy fix is to change this from Strict to Loose, however if you are unable to login to the admin panel this may be a little harder to fix.
The easy/quick way to fix it via the command line is as follows. Log in via SSH, and edited the file /var/cpanel/cpanel.config
Change the line that says
and make it
If you need any further help, pop in a support ticket