If any emails were sent, then it is possible to find out the IP address. If not, then you will have to ask Yahoo! Mail for help, as they will certainly be able to tell you any information they have about the time, IP address, and what messages were read from various times the email account was accessed. If it was done from a public place like a library, then it would be much harder to prove who did it. You would have to check the library to see if they have logs of who was on which computer at what time. Check out the
Yahoo! Privacy Feedback and the
Incident Form for when people try to assume your identity.
I'm not sure if there is any precedence for legal consequences to email snooping, but I do know that it is illegal.