If the suspect is still present, you should call the police on 999. If the suspect has left, you should contact your local police force who will take crime details either over the phone or online. It is important to know the time or times it happened between and the size of the window smashed.
You can contact your local force by calling 101 or using the contact details for your local force which can be found at Q727.
If the house is a privately owned, you should take steps to replace the glass and secure the house. Details of approved glaziers can be found online; the police will not provide recommendations. If the property is a council house, you should contact the council to repair/replace the window.
To find your local authority, please see the link in related information.