There are many, many possibilities. Unfortunately. And with such a terse statement of your configuration it's virtually impossible to give you more than a guess. But here's one that I experienced when I set up my wife's droid.
Her computer, running Outlook (not Express) was configured to download email from the email server every few minutes. When it did so, it deleted the email from the server. Not realizing that, I set up a notification to her new Droid when email arrived at the server. She was notified but when she went to check the email it had mysteriously disappeared. By that time it had been downloaded to her computer.
i have had my droid for a while and never had any problems getting my email before. did go to setting and it says that my email is synced but it will not display on the phone