How does Microsoft know that someone else is using my Hotmail account?

http://ift.tt/1iZHv9S

Some days ago someone hacked my Hotmail. I received an email from Microsoft that someone else was using my account. How do they know this?

And then a few hours later it was impossible for me to go into my own email account. I needed to fill in a form to prove that the account was mine and then I needed to reset my password.

Did someone else really did into my account?

I always login from my own computer

How do they know? And how can I know who was it?

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