When you say "only see last three days" - is that just your Inbox? Or are your Deleted and Sent items gone? Do you have any archived folders? Is there mail in there?
I don't know if this is contingent on whether or not this is an IMAP or POP3 account, but, here's a shot in the dark:
You likely have a file called "[your_email_address@yourISP].ost" in your \AppData\Local\Microsoft\Outlook subdirectory.
Before doing anything else, make a copy of that and put it somewhere safe - that may or may not be an archive of your emails, and the more you sync, the more you risk damaging it.
Most likely, if the ISP did something funny it would have synced the funniness back with your machine, making it think it's OK to delete those messages.
Now, here's where the luck comes in - do you archive your machine? Go to the archive nearest before (but not after) you observed the deletion, and restore that .ost file. Now rebuild Outlook using that as the mailbox.
You may not have everything, but....