It might be too late, but had the same issue, after battery replacement I saw "Bac L" in the to bar (for back light) and seconds counting on the main counter.
None of the front buttons were responsive.
Hard reset helped. To do this you need to hold teh middle butttin and then press reset button on the back side with a paer clip or sim card pin opener.
Note that the reset hole is not in the middle of the back, but on the cover, to the left, under the screw.
You will rose all the data that is stored in the device, but at least this thing starts to work.