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 it says it is valid, try the following as root via SSH:

/etc/init.d/firewall stop
rdate -s rdate.cpanel.net
/usr/local/cpanel/cpkeyclt
/etc/init.d/firewall start