Emails are disappearing on their way to MS Outlook

I run a small 1-person business from home and my business has recently been decimated by the recent discovery that many emails are just never reaching me! I put in a query with my ISP provider and even their responses never reached me!! Their investigation has showed that the emails reach their server but disappear when they are then downloaded to MS Outlook on our computer. We have no rules or junk filters set up and we have not changed anything to suddenly start this. We had no virus software/filters that may steal them. We just put a good virus software on the computer and nothing relevant was found.
These disappearing emails means that queries for new business often never reach me and it has cost me a fortune!!!!
Any ideas???