Hi Kathy, I think you should call Yahoo! if you are unable to log in and find out what happened. The number for the passwords department is 1-408-349-1572 Option 2, Then if in fact it is that someone has gotten your password, here's what you'll have to do as the agent will tell you the same thing;

I have had a situation where my account had been hacked too, but I talked to three people at Yahoo telling me the only information that could not be changed was by Birthdate, and my secret question and answer. It was described to me that if I provided the correct information at registration like they said when I "agreed to the Terms of Service", I shouldn't have a problem. Luckily enough, I was able to get my account back after all of the following:

Yahoo asked me to e-mail [email]account-security-help@cc.yahoo-inc.com[/email] (as there is no phone support for that department... ) put my Yahoo ID in the subject line and be specific what occurred, the last time I was able to log into my account. and wait for an automated response which didn't come like they said within the first 6 hours, so I checked my spam folder (bulk) for some and there it was, so I waited for the second e-mail which asked me for all of my information, after the second try, I got my questions right, the first time I didn't put the right zip. So when I replied the second time, I put in like 5 different zips I was at and as many different ways as I would answer my alternate e-mail addresses and my secret question and answer. Hopefully you all have good luck like I did.

I asked if I could get information on how I can find out who did this, turns out i need a court order or subpeona from my local authorities or a lawyer in order to request that information. because get this, if I was to go find this individual and hurt them, Yahoo said they might be held liable for any damages done to this person, so that's why I have to go through the legal rout. I know twisted, but that's what they said the procedures were.

Good Luck