mysql.user table lost

Do you have any cPanel backups available? If you do not, you might check /var/cpanel/databases to see if a list of grants is available there for each user to recreate the users in MySQL command line. The files for grants will look similar to the following:

Code:

root@host [/var/cpanel/databases]# cat grants_cptest.yaml
--- 
MYSQL: 
  cptest: 
    cptest: 
      - GRANT USAGE ON *.* TO 'cptest'@'208.74.124.109' IDENTIFIED BY PASSWORD '*D9D9A5D88C42480E4B782E9DD9799FB81DEBB4DF'
      - GRANT USAGE ON *.* TO 'cptest'@'host.daniellewallace.com' IDENTIFIED BY PASSWORD '*D9D9A5D88C42480E4B782E9DD9799FB81DEBB4DF'
      - GRANT USAGE ON *.* TO 'cptest'@'localhost' IDENTIFIED BY PASSWORD '*D9D9A5D88C42480E4B782E9DD9799FB81DEBB4DF'
PGSQL: 
  cptest: {}

If you do not have /var/cpanel/databases/ with the grants listed nor have any backup files, you are going to have to recreate each database user from scratch. I would highly suggest contacting your datacenter, NOC or provider at that point.