My Sent Emails has Disappeared - Windows mail

Hi every one,

I use windows mail,
My problem started when I needed to check some sent emails. when I went to my sent items I realised 3 month (May-June-July) worth of email has completely disappeared from my sent items, whole lot. The rest are there.
My inbox is fine.
They are very important emails to me.

I do Not delete any email inbox/sent ever.

PLEASE HELP:

1. where are they?
2. why these has happened?
3. how can I retrieve them back, if possible?

I really appreciate any help,

Regards