My Apple Watch only shows the time and a small lightening bolt

It is is Power reserve mode. See the 'Use Power Reserve section in:

Check your battery and charge your Apple Watch - Apple Support'

To turn off power reserve mode,press and hold the side button for about 10 seconds, until you see the watch restart and the Apple logo appear.

Keep in mind that if your battery is almost completely empty, you might need to charge the watch before it will restart normally.