If a whole sales channel has vanished from your account — gone from your list entirely, not just showing disconnected — it's almost always an access or login issue, not a technical fault. Two things to check:
1. Are you logged in with the right email? (most common)
The channel may be on an account under a different email address than the one you're currently logged in with — for example if you (or a colleague) set it up under another email, or you have more than one login. Log out and back in with the email the channel was set up under, and check whether it reappears.
2. Is your login an additional user on that channel?
Access in Link My Books is granted per sales channel. If the channel sits under someone else's account, your email needs to be added as an additional user on that specific channel before you'll see it — and only the account owner can do that. Ask them to add you: see How to add additional users to your account.
Still can't find it?
Contact support — we can look up which account and email the channel is under, so you know where to log in or who to ask.
Note: this is different from a channel that's still in your list but showing as disconnected — that just needs reconnecting. See How to Reconnect a Disconnected Sales Channel.