i dont believe your account has been hijacked. however, i also dont know exactly what the problem is either.

so i would suggest you see if any settings have been changed in your browser... make sure you are going to the correct yahoo page the first time you try (just incase its not the yahoo page) and if nothing changed, get a different browser and try that. if the other browser doesnt work, i could say almost definatly that its server related and hence Yahoo's problem.

recommended browser: Firefox......