Hello, I suppose you're using Teams desktop app. Perhaps it's been some time since you signed out from Teams? Do that as a start (from the top right corner) as it will refresh things. You can also go to Teams online to verify the existence of the channels and then you know for sure if it's a cache related issue.
Teams free or business subscription? For the former there's no official support but if you're on the latter I recommend opening a support ticket with Microsoft. You'll need to reach out to your IT admin if you're not an admin yourself.