If one of your employees set up your YouTube Account, and you no longer have access to the YouTube Account, you have various options:
try resetting the password (you can do so here or on this page). The reset password link will be sent to the primary email address on the Account.
if you don't have access to the primary email address on the account you can attempt getting in contact with your ex-employee and requesting to re-gain access to the Account. You can try to do so by sending them a private message to the YouTube Account in question.
(Note: you'll need a YouTube account in order to send a private message. Learn how to send a private message here).