WHy is my Popmail now closing after 2 seconds- its unusable?

Propblem just started this morning...when I switch from my Yahoo mail account to my popmail account, my popmail account inbox shows for about 2 seconds and then reverts to the yahoo mail account, making my popmail USLESS. Whatis going on, how do I get the Popmail inbox to stay open so I can read the mail there??