Watch turns off and screen goes black although charged. And it doesn’t stay charged even though plugged in.

My watch sits there plugged in and loses all its charge so that when I go to put it on it’s flat.
At other times it says it has charge - it said 79% today. But then I took it outside, found the gps signals, started running and 20 mins into the run the screen went black and it stopped responding. The other day it stopped working just after it found the gps signals. 

What to do??? 
My old Polar RS400 was still going after 2 years sitting around,  so I had to use it with a chest strap! Weary But it worked! 

Thanks

Sue.