I just went thru the exact same thing.
Was on a walk, got some very light rain, and when I looked at my Alta HR...same thing. Tried the reset numerous times, no joy.
I finally just let the battery totally run out, and left it alone for a few days. Put it on charge, and now, it's ALMOST back to normal. The only difference that I notice is that the green sensor lights no longer flash, they're just solid. But, it is registering my heart rate. I'm guessing the battery life will be a bit shorter between charges.