Disappearing E-mail – inbox&sent folder

I’m using Outlook Express 5.5 as my email client. and periotically, 3 times in the last 2 months, when I open my inbox I discover that all my emails have disappeared. I have run a virus scanner and have determined that I am not infected with a virus. I then checked my email options and could not see anything that would cause this to occur. I’m not currently using any filters such as hide read messages. I am perplexed as to what is causing this. Where are my email messages? lol