I had this problem in the past. But it was due to the introduction of a new animal in the house. Have you changed something in your household recently that coincides with when this problem started?
tools -> internet options -> programs then select yahoo mail in emails area. If it's not there then it's not installed then you'll have to install it again from yahoo.
Well... the icon you click on for your web browser should tell you. It's either internet explorer or firefox... unless you have something else installed.