Apple Watch series 1 went blank

It looks like the battery may need charging (or it has become defective). Check your charger is working correctly, test with a different magnetic cable and power adapter if you have them available. Then see If your Apple Watch won't charge or it won't turn on - Apple Support

Try a reset by pressing in the crown and side button until you see the Apple logo, then release both.

If it still will not turn on it could be you need a battery replacement.