I recently replaced the alternator a few days back, everything seemed to be working fine for the past three days, until this morning I park at the store, go in a few minutes come back out and the car won't crank. Replaced all the fuses in the fuse box and had the battery check, it was good. Tried jumper cables,...nothing. The engine will not turn over at all. Power windows all work fine,..the charge seems to be there, but when I turn the key, absolutely nothing happens.

Any advice?

Check the start inhibitor switch on the gear selector.

If it is the neutral safety switch it should start in neutral.

You might try running a jumper cable directly to the lug on the starter where the large red wire connects. The starter solenoid is activated by +12V from the ignition switch so that wire - blue/yellow I think - should also be checked.