iwatch screen went black wirh white line at top

That's likely a hardware failure. To be sure, let's try a Forced Restart:

Press and hold BOTH the digital crown and power buttons until the screen goes black and keep holding until you see the Apple Logo. This can take several seconds, so keep holding. Hopefully, you'll be prompted to enter your passcode, but if you are not, the screen or watch has failed and an appointment at your Apple Store Genius Bar is the next step.