This is a network issue, not a mIRC issue.
When you connect to the network, do you receive a notice that indicates that the nick you are using is registered and asks for a password? If not, then the nick you are using isn't registered, and that channel now requires you to have a nick that is registered on the network.
If your nick isn't registered, the normal format to register a nick is /msg nickserv register <pAsSw0rD> <valid e-mail address>
<pAsSw0rD> may be case sensitive, and may have a minimum number of characters that is required. A valid e-mail address is required on more and more networks, as the registration process is two step.. 1) The request to register the nick (as I posted earlier), and 2) Sending a confirmation code to nickserv, with the confirmation code being generated and e-mailed to you at the address you provided.
Note: not all networks require the confirmation code, and some don't even require the e-mail address, however, as stated, these networks are getting fewer and fewer.
For further assistance with registering your nick or other network services, check with the networks help channel, usually #help
