Assuming you had a Hotmail account? They are transitioning people over to outlook.
It didn't just "suddenly appear" It asks you if you want to make that move and you have the option of yes and no. I know because it asks on mine all the time. That's why it's so important to read first instead of just ya ya click click click. Once it's moved over I don't think you can switch it back. But a quick google search which im far to lazy to perform would give you that answer.
"Why can't I switch back to Hotmail?
Over time, Hotmail will be phased out and Outlook.com will be the free email service from Microsoft. As part of this transition, we've removed the option to switch back to Hotmail."