The most common cause of all networks suddenly not working and giving that error message is an update to a firewall or antivirus program, assuming your entire internet isn't down. Sometimes, an update to firewall or antivirus can turn on an IRC blocker, or else it can disable the internet permissions for mIRC. So you should check your settings in your firewall and/or antivirus. If you want help with that, please provide the name(s) of what you are using.
Also, if you move mIRC to another folder or install a new copy of it, your firewall may block it from connecting to the internet, so keep that in mind as well.