Firstly, as others have already said, don't boil milk in a kettle. But I guess you've learned that lesson pretty well at this point.
In most offices I've worked at, the kettle tends to be a pretty cheap piece of equipment and needs to be replaced fairly often. It isn't going to be a problem for the company to get a new one, and they will be unlikely to make a fuss about it unless it was brand new or unusually expensive.
You will need to confess what happened and take responsibility for it. Possibly you'll be asked to pay for the replacement kettle. There's also likely to be a bit of mockery from your colleagues. If that happens, run with it: turn the banter around and laugh with them; this is a minor thing that is easily sorted out, so don't let it turn into a big deal or spoil your relationship with your colleagues.
One final note: People like their tea and coffee at the office. They're probably already missing the kettle. If there's no kettle for a long time, it may generate some resentment. Go and talk to your manager or the office facilities manager as soon as possible. The quicker you get this sorted out, the less of an issue it's going to be for everyone.