Mercury Grand Marquis (1996) won’t start

Hi all! I’m having trouble diagnosing my situation,
so here it is.
I inherited this car from my grandfather, and boy do
I love it. Panther body, smooth rides, large seats-
wonderful car, and the gas mileage isn’t the worst
for its time, either.
Everything worked fine in the past. It could act a
little jumpy sometimes- doing little “hiccups” while
driving- but I’d just take my foot off the gas pedal
for a second, ease back into it, and all was right
with the world. I’ve also been told that the engine is
really loud- but that’s coming from my mother, who
has driven hybrids for most of her life.
Fast forward to last week. I don’t drive it much-
maybe twice a month- and when I try to start the
car it won’t. I turn the engine, feel a very silent click
(although for all I know that’s just the key turning
the ignition switch) but nothing else. No sound, no
“turning” or whatever the lingo is, and weirdest of
all, my lights, AC, and radio all blare to life.
Of course at first I thought it was the battery, so I
bought a trickle charger and charged it overnight.
That didn’t work, so I tried trickle charging it AND
heating its block for four hours prior to starting.
Again, no luck.
What could it be?