All of my emails that were in folders have disappeared.

Hey emailsgone1,

Welcome to Apple Support Communities. We see you're experiencing an issue with missing emails. Below we've provided some suggestions that may help:

• If you use more than one email account in Mail, make sure you’re looking at messages for the correct account.

• If you sorted messages, they may be in the list but not in the order you expect.

• If the missing emails are from a specific person or group, check whether you blocked messages from them.

• If you use rules, they may have moved messages to other mailboxes. Try searching for the messages.

• Check the account’s Recovered Messages mailbox, if available in the Mail sidebar.

• Quit Mail, then reopen it.

If the problem persists, you may need to rebuild a mailbox.

You can also review the steps anytime here: If emails appear to be missing in Mail on Mac - Apple Support

Take care.

Maybe the folders are just hidden.

Where are they - inside one account, or under "On My Mac"?

In each case, you may hover the mouse pointer near the name of the account (or the words "On My Mac", as the case may be), and click to disclose: