First, check your whole property to ensure there's definitely no fire. Some fire alarms will have a ‘silence’ button which you can press to stop the alarm. If it continues, remove and replace the batteries as the smoke detector may be alerting you to low battery levels.
If it still continues, replace your smoke detector and dispose of the old one correctly. Smoke alarm sensors have a limited lifespan – often 10 years – after which they need to be replaced even if the battery is fine.