You're probably joining an empty channel. If it's a channel you've used previously you may have been on a different network. #mIRC on UnderNet is not the same channel as #mIRC on DALnet, for example.
If you can't remember what network the channel was on you can try searching at
SearchIRC or
netsplit.de.
You can also use the
/list command to list the channels on the currently-connected network.