How to fix a red exclammation mark

Hi, after getting a new iPhone and trying to pair up with my watch, my watch (series 2) has crashed, and has a red exclamation mark showing on the screen. When I try to restart i get the apple symbol briefly then it returns to the exclamation mark. Any ideas what has happened and how I can resolve?

(my iPhone is not picking it up at all so it hasn't paired before crashing). thanks!