Hotmail account inbox emptied - Why ?

I have two hotmail accounts and both are working fine on my iMac 24" under Ventura 13.1.

Both were also working fine on my iphone 13 under iOS 16.2.

Yesterday, the contents of the Inbox on one of them was empty but Sent etc were still there. This applied to the Mail on iPhone and thro the Outlook app.

I have deleted and then re-added this mail account to the phone but it still does not fill up with the contents of the Inbox (or indeed the Sent etc as obviously this was emptied when I deleted and re-added the account)

Questions:-

Why did this happen ?

How can I get this hotmail account on my iPhone to contain all the info that the same hotmail account shows on the iMac(i.e. the contents of Inbox, Sent etc ).

Thanks for your help.