Iwatch LCD malfunction
My I watch got LCD mulfunction i dont know the reason why, after my alarm pop and vibrate.
Apple Watch SE, watchOS 7
As first step, Forced reset the watch to eliminate any software lock-ups. Press and hold both the side button and the Digital Crown until the Apple appears (we hope) then release and let it reboot. (If there is a screen problem and the apple doesn't appear within about 30 seconds... there is a problem!)
If reboot doesn't fix it you need help from Apple Support to arrange repair. Use (link) Get Support to arrange service for your watch.