First, check the license on http://verify.cpanel.net/ to ensure it is still licensed.
If it is not, please contact who you purchase the license from directly.
If your license lists as active and you see this error when trying to access cPanel or WHM: License File Expired, you will want to ensure that port 2089 is open for outbound TCP traffic.
After verifying that port 2089 is open, check to make sure that this command gives the IP that you expect: lynx -dump http://www.cpanel.net/showip.cgi. If that command shows the expected IP, run this from the command line as root, on the server: /usr/local/cpanel/cpkeyclt.
# rdate -s rdate.cpanel.net