emails in my folders have disappeared. can I recover them?

Settings, Mail Accounts.

Tap on the account

Is there a "Mail days to sync" that is set too short?

Is it Gmail? Yhey have their own rules.

Whoever the email account provider is :

Sign into the account in a browser, preferably on your computer,

Check if they are there.

Lost sub folders on mail a/c

under inbox or under other folders you created?