If you are able to, first check your consumer unit (which used to be called your fuse box) and make sure your supply hasn't tripped. If you have a prepayment meter, please also make sure you have sufficient credit on the meter (Click here for more information on prepayment meters).
For information about if there is an outage at your site, please go to the Service Status page and if there is no record of a fault, you will be able to raise the fault on the form on that page. For emergencies please call 020 3488 1411.