My Apple Watch is getting charged but not turning on.

My Apple Watch is not pairing with iphone 6s plus iOS 11.3.I tried frequently but failed.seve days later when I charged my wa but it becomes hot And not turning on.now it is showing to be charged and turning green but when unplugged showing red and not turning on.What can I do now?Is there any battery problem?how can I turn the Watch on?and how to pair with updated iOS??