Why do I only have lightning bolt & time on my watch face?

Hi

Apple Watch displays the time and a lightning bolt (and cannot be used for any other functions) when it has entered into Power Reserve mode, which happens automatically when the battery reaches a low charge level or when it is enabled manually.

To turn off Power Reserve mode, restart your Apple Watch: press and hold the side button until you see the Apple logo. You may need to charge your Apple Watch before it can be restarted.

More information:

Check your battery and charge your Apple Watch - Apple Support

If your Apple Watch won't charge or it won't turn on - Apple Support