Yahoo blocked my access and asked me to change my password,which I have done . How do I know if the request from Yahoo is genuine.

It probably wasn't. What is the exact sending address?

Your entire computer and all accounts could be in danger.

Did you follow a link in the message to change your password? If so you were probably phished. Log onto Yahoo, immediately and change it again. Change your security questions too.

You never use the link provided in any email. Also close that browser and open it from your own link.