Watch stuck on power logo screen

Received the newest update (4.20) and went to install it. Watch stayed on 'Restarting...' for several minutes and nothing happened. Now it's stuck with a power symbol on it, and I can't restart it or do anything to get rid of it.

Backlight still works and notifications are still coming through (and I can reply to emails, texts etc), so I don't know what's going on! Any suggestions?!!