I have a Mamiya m645.
This is the old one, before they came up with the swoopy viewfinder.
(model "J"?)

I got it 2nd hand last June, and it worked quite well
(except that the thing that automatically resets the frame number
when I finish the roll doesn't work).
I used it off and on through the summer and fall.

Then a couple weeks ago, I went to use it,
and the LEDs of the "exposure assist" (match the "needles") didn't light up.
I couldn't think of any good reason why the battery would have run down,
although there probably is such a scenario.

Anyway, I bought a new battery, and tonight I put it in.
Still no LEDs!
And, more specifically, pushing the little "lamp-test" button on the
right-hand side does not light up the little "battery-OK" LED
on the left-hand side.

Any ideas on what might have gone wrong,
or what state it's in now?