e-mails disappeared

Hello,

I have an ipad2. I have 2 mail accounts set up on it. I just opened up the POP account and found that about half of the older mails have disappeared. I did a restart. No change. Then I did a reset. Still no change. Seems the mails are just vanished into thin air. Please advise if these are retrievable and/or if there is something I need to change in my settings to avert this occurring in the future.