is hacking yahoo mail is as easy as sending yahoo mail.........?

A giant company like Yahoo should be secure right? If an incident on my personal Yahoo email account last week had not happened, i could probably have doubts that Yahoo can really get hacked. I havent heard of it before, until i got victimized. My account got hacked and then deactivated. I don't really know how, i was just shocked to find out i can't accessed it anymore because it got deactivated. How come would it get deactivative when i use it daily. Nobody knows the password except me, im very sure of that. Geez! those hackers are so brilliant! I was also thinking, maybe i was able to logged in in a phish site the day before that's why it happened. One should be very careful on checking websites they log on.