Folders not showing in Outlook

My partner uses Outlook for his emails and has about 4 different accounts that are normally shown as folders. ie his personal email, his business email, his social email and messages are automatically sorted into those folders. When he's gone into his mail today, it's gone straight into his personal email and none of the other folders can be seen. I know they're still there because when you send and receive it does it to them all. I've tried everything go get it back to how it should be but nothing has worked