There's hundreds of IRC networks, each made of one or many different IRC servers. I suggest you try some servers out of the "default" servers list, most should be working:
- if mIRC is running at the moment, quit mIRC
- download a new serverlist: http://www.mirc.com/servers.ini
(right click - save as: servers.ini)
- replace your current servers.ini with this file and restart mIRC.
If you have problems to locate the folder containing your current servers.ini, type in mIRC: //echo -a $mircdir