Hi Stephen,
maybe this helps as I was in a similar situation just recently. Mails
that I get daily suddenly didn't show up in my inbox anymore although
I could see them on the server or when opening my inbox from a
colleagues Outlook.
Have a look at: "Extras" => "Options" => "Junk E-Mail"
If you have set this setting to "High" AND checked "immediately delete
Mails recognized as Spam and don't put them in the Spam folder" (or
similar, I have the german version of Outlook) at least uncheck this
box. The mails then should show up again in your Spam folder or, if
you set the settings to "Low", they should appear in your inbox.
HTH,
Andy
P.S.: Don't say "I have set these settings ages ago and only recently
the mails disappear". Same situation was in my case, these settings
("High") haven't been touched for months and suddenly certain mails
didn't get through to my inbox anymore!