It turns out I also developed this problem, or a similar one, though in my case it was stuck in Off. Wouldn't do anything!
Here is the way that turned out, you may have the same thing (from the thread I started on the issue)
RESOLUTION
So in our last exciting episode, I had dropped the bike off Saturday afternoon at my LBS. They had not seen this one, and said they would call Bosch on Monday and report back.
Sure enough, Monday (yesterday) around 11AM they had the thing fixed. During a call to the Bosch technical people, they removed the controller - a couple turns of an allen screw on the bottom side of the unit - and discovered some unspecified amount of dust/crap/crud on one of the contacts within. Cleaned it off and Shazam! Back in business.
This will probably never happen again, since I don't ride this on dirt much, but it doesn't happen to the mountain bikes either, so this one was a fluke. However, if it ever does, out on the road, a quick sacrifice of a CO2 canister from the tire repair bag would most likely blast it clean again just by shooting air into the edge of the controller without removing it. This stuff gets COLD so just a tiny shot would likely be a Save.
The thing comes apart after removing from the handlebar just by poking the end of the allen wrench into two little holes on the underside of the Plus and Minus buttons. They pop right up and expose the contacts. I'm not convinced I would mess around to that extent unless it was an emergent sort of situation... five miles up the side of a six mile mountain, perhaps.
By "controller" I am referring to the assist up/down selector on the left handlebar. This is on a Trek 700+, Intuvia setup.