You will need to show the messages you receive when trying to connect for us to be able to help you.
mIRC registration is lifetime. That's what you pay for. Registration on a network is *not* lifetime unless you are always there. Most network registrations will expire in 30 days if you don't connect within that time period. Network registrations are free. Each network can have a different process for registering, changing passwords, or having your password sent to you. You should check the #help channel on the network using another nickname for information on how to do so on your particular network.