I put the watch on the charger

That is power reserve mode;

To turn off power reserve mode,press and hold the side button for about 10 seconds, until you see the watch restart and the Apple logo appear.

Keep in mind that if your battery is almost completely empty, you might need to charge the watch before it will restart normally.

I am pretty sure that is because it is charging and so it may have run completely out of battery so like how when the iPhone runs completely out of battery it shows a battery logo so I think it is like that.