it's telling my nickname isn't registered, but i know that i did. for some reason, i can't find the email that was sent to my after i registered. and i'm not sure what my password was, but i think i know it. what i can do to fix this?

Nickname registration does not exist on all IRC networks, and furthermore, it has nothing to do with mIRC itself. You need to ask the network staff (usually found in #Help) or go to the network's website for help with such issues.

Out of experience, make sure that the email has not been caught in your junk mail folder and wait a further 24 hours for the email - their mail servers may be slow or down. Try identifying to your nickname too with whatever command applies to the network you use.

To reiterate though, for further help with this issue, you need to go to the network's #help channel or equivalent.

Regards,

In many cases the following command helps:
/msg chanserv help

Otherwise as said above ... go to your network's webpage or talk to the network staff.