Let me explain a bit:
There are many many Networks. Each network has its own servers (you use a program like mIRC to access networks only thru THEIR servers). Each network has its own channels, and the same NAME of a channel may be on many networks. For example, #mIRC can be found on DALnet, EFNET, Undernet etc etc, but each one is a different channel, with different founder,ops, and users.
The channel name you are looking for may be on many different networks, and the one you connected to this time may not be the same one you connected to before. In other words, you could be in a channel with the same NAME, but not the same channel you were in before because its on a different network. It could also be that the channel just happens to be empty at this time. Not all channels have ppl in them 24/7. Another thing to be sure of is that you are typing the name of the channel correctly.
Just to complicate things further, there are things called netsplits. Suppose you are on a server named dudly on SomeNet. The other ppl in that channel are on a server named quentin. For any of a bunch of reasons, dudly and quentin stop communicating for a bit (split). the ppl on each server are still connected,and still in the channel, but they cant see each other until dudly and quentin reconnect to each other.