I don't know. All of my mail folders are present. Login to your email provider's website to see if the folders are present, and if so, try signing out or deleting the mail account from your Mac, and then reconfigure the account on it.
Don't fret! Same thing happened to me. Go to Mail > Preferences > Accounts, and then uncheck the "Enable this account" option and then re-check it. Your folder structure should be restored.
Thank you ByranBuff, your instructions were awesome! You are brilliant and thanks to you I avoided a heart atack tonight and I can actually get some sleep!! Thank you, thank you, thank you!!
THANK-YOU for the tip:
"Don't fret! Same thing happened to me. Go to Mail > Preferences > Accounts, and then uncheck the "Enable this account" option and then re-check it. Your folder structure should be restored."
It worked for me!
Thank you this fixed it for me too. I had folders from hotmail and icloud. Such a shame that apple doesn't test updates properly these days though - just like Microsoft.
Thank you! Unchecking and then checking "enable this account" worked great. It was immediate and worked - unlike the recommendation I found in Apple Help!