When no human being ever answers it's usually because some more senior human being in the company has screwed up. A mere router failure or other hardwarefault fault would get fixed in a few hours, and you'd get a chipper response telling you how effective their technical backup was. But after all this time and still no data.... somebody at Yahoo has reasons for not saying anything. Maybe the manager whose job it was to ensure that the backups worked? Maybe the bean counter who ran out of beans to pay the server farm which hosted your emails. Who knows?

Yahoo is free to users. As we don't pay them anything we have no rights where our data are concerned, so it's safer for Yahoo never to say anything in a situation like this because anything they DID say could get pounced on by a lawyer as an admission of liability.

The hard lesson is this:- always make backups on your own PC of any data you entrust to Yahoo. It's boring and time consuming and a nuisance, but I'm afraid the alternative may be an exciting problem you can do without.