Background: 1990 Mazda Protege, very well maintained, very clean
under hood. Last night it started as usual. I drove it to the dumpster
at the edge of my property to dump some trash, stopped the car,
and cut engine. When I tried to start it back up, I got the usual
click of a dead battery; digital clock very dim etc.

Went to NAPA today and bought topnotch battery (the same
kind that came in car when new). I did accidentally install
it backwards (neg lead to pos post and pos lead to neg post),
got some sparks, but the battery is now securely connected.
The lead clamps are pretty clean and don't appear damaged.
But no power whatever, clock dead, nothing.

Ideas?