Firstly, check the panel to see if there are any fault lights showing. This will give you an indication as to whether there are any issues with the panel. Depending on your system, it may also tell you the device location that may have caused the alarm to sound. If there is nothing showing, reset the panel.
If you can't reset the panel and it goes off again when you try, does it sound immediately or is there a delay of a few seconds? If your panel sounds immediately, it's an issue with a call point (red box) that has had the glass broken / been activated or has no electrical supply.
If there is a delay, it is a detector that has activated the alarm. Check areas where detectors are located for any possible causes of activation.
If you are still unable to reset your system, call your fire alarm company who will assist you accordingly.