Flight mode is on. Flight mode cuts off all wireless communication from your watch. You can still use it, but you can't sync your data with the Polar Flow mobile app or use it with any wireless accessories.
Do not disturb mode is on. When do not disturb is switched on, the watch will not vibrate when you receive notifications or calls (the red dot will still appear at the bottom left of the display). Also, the backlight activation gesture is disabled.
Alarm is set. You can set the alarm in Settings > Watch settings.
Your paired phone is disconnected and you have notifications turned on. Check that your phone is within Bluetooth range from your watch and Bluetooth is enabled on your phone.
Button lock is on. You can unlock the buttons by pressing and holding the LIGHT button.