In most cases this is because you have tried to connect to a server that doesnt exist (perhaps you mistyped it) or is down. By using your favorite search engine, find the website for the network you are trying to connect to and look at their list of linked servers. Make a note of a few of them, (Since things change, the server list that comes with mIRC and even a networks website can be inaccurate at times)
and try to connect to them by typing in your main mIRC window:
like: /server dudley.irc.net (thats a fake one so dont try it)
Since it could be the entire network is down, try connecting to a different one to see if it works.
Try turning your scripts off.
Its possible that you/your isp are having dns probs, and you may need to connect to the ip rather than the domain name of the server. Like: /server 123.456.789.123
You can try getting that from /dns theserver.info.here (altho it may not work if you are having dns probs) or try checking their website.
It could just be the server is full, try another one
Some networks, like DALnet, have set their "round robin"/ Random (irc.dal.net), to not resolve correctly, so you will need to try connecting to a specific server. Again, check their websites or various sites on the web that keep track of server connections. (one site for DALnet servers is here
and another is here
Another possibility is that you have mIRC or the server ports blocked in firewall/router (yes, that includes the firewall in winxp). Server ports: 6660-6669, 7000-7001 and port 113 for identity
If your connection is timing out, you could try in mIRCs options file(or view, depending on version)/options/connect/options click on RETRY and increase the line "if not connected in ___ secs" a little bit ( i use 60 secs)
For details of other connection errors, please see this thread
If the prob is 10053 (software caused disconnect) then do a search here, all forums, expand the date range, using 10053 as search term. This seems to be very common (especially during dcc for xp users). There isnt any one-solution-fits-all, but you will find things that have worked for others. More specifically, see if this sticky thread
helps you out, it has almost all advice we can give.