Additional team and channel-related features and issues within Microsoft Teams for business
Most likely fix, click "Chat" in the left sidebar. In the new Teams, Chats and Teams/channels have been combined into one sidebar item, so if you are looking for a separate "Teams" icon to find your channels, you will not find it. Microsoft merged them, but you can get it back in settings if you liked it.
Inside Chat you will see your one-to-ones and group chats alongside any teams and their channels. Click a team to expand it and the channels sit inside.
If a specific channel is missing, it might be hidden, click "see all channels" at the bottom of the Team