Why can't I join a channel?

There are several things that might prevent you from joining a particular channel. In most cases the server will tell you the reason, but some clients do not correctly display that information to you, so it's useful to be able to check for yourself. Some of the common causes, in no particular order, are:

If you've not been in the channel in question recently, it's possible that you're affected by a wide-ranging ban, so pay attention to any bans that include * or ? wildcards. Some bans may be extended ban types; you can see more information about these here.

If you find a ban which matches you but you feel wasn't intended to keep you out, then talk to one of the channel operators to see about having it removed.

If you're still stuck, feel free to ask a staff member for help in working out what's preventing you from joining.

Why can't I talk in a channel?

The two most common causes for this are either a mute ban -- use /mode #channel b to see the list, look for bans starting with m: and then check the advice above regarding bans -- or mode +m, which prevents anyone who isn't voiced from talking. The channel may also be set to mode +M, which prevents unregistered users from talking. If you're still unsure, again, feel free to ask staff for help in working out what the problem might be.