Why Did Emails Disappear From My Ipad 2

I rebooted my iPad 2, went into my settings and reinput my password but am still not receiving emails into my main account. I can access my mail via the Yahoo app but not the web interface.

This is the second time in as many days that this has happened. After rebooting and reentering my password the first time, my main account emails reappeared. My actions may have only been a coincidence since repeating the process has not corrected the problem this time.