I had this issue after spending the better part of a day removing severely corroded batteries. I was devastated to find it didn't turn on after all the work I put in.
I looked at everything more closely and noticed a key thing. The screw holding the logic board into place is the negative terminal for the batteries! When I put everything back together I didn't tighten that enough *and* I had a bunch of dust and crud in the screw threads on the end cap as a result of the cleaning. I gave it a good rub with a paper towel and Q-Tip and voila! Back in business!