I have seen part of the problem but luckly don't get enough mail there to loose any.
In the mail there is a help menu top right and if that is selected you can contact technical support which maybe able to help.
I have done some checking and they are rather light on info and there so far doesn't seem to be much in the way of places offering help.
One thing I have noticed over time is yahoo seems to be updating the mail a lot and I have seen some strange things happen.
One thing whch may or may not work is changng the mail back from the new version they have had a for a while now back to what they call classic mail. This may allow the mail to be seen again.
I will do some more digging and see what I can find the in the way of links to help.