I know there is a certain irony in that if you have a problem, you may not be able to read this, but it is here for reference...
There are numerous reasons why you may not be able to connect, these include:
1) Software errors
2) Computer error
3) ISP errors
4) Other network errors
5) Server errors
6) You've been blocked in the firewall
7) incorrect credentials
The first two are possibly the (or one of) most common causes of connection problems. They can usually be cured by exiting and restarting the application, or even rebooting your computer.
Sometimes there are problems within your ISP - there's not always much you can do here, except wait until they get it sorted. This can usually tested for by trying other websites/servers to connect to. If they also refuse to connect, then this is the possible reason.
There are times when a connection somewhere between the ISP and the server goes wrong. This may manifest itself in only a few servers not responding. Again, there's nothing you or I can do but wait for that to be fixed.
The server, while reliable, is still a computer and does go wrong - we are usually aware of this and will try and get the problem fixed as soon as possible.
You may also have been blocked by the firewall - if you have, you should get a short message in your browser with brief instructions on how to get unblocked.
And finally for now - you may have the wrong username/password. Remember that these are often case sensitive (which means that Name and name are different!)