Recently washed? This was a common problem when the RC1 unit (power on, assist mode control) or its connector in the down tube gets wet. Often the fix was to simply let it dry out. It happened to me once.
The solution for me, was to apply contact grease and tape the connector. I also taped the RC1 360 degrees around the seam connecting top and bottom. This when done neatly was hardly visible. I never had another occurrence, and rode the bike almost 5K miles.