All Gears Gone?

No forewarning. No problems or clues at all prior. It was running great. All fluids checked and good before I left. Driving down the highway, 75mph, approximately 250 miles into the 360 mile trip I was taking & all of a sudden it was like it had seamlessly slipped into neutral. I tried moving the gearshift into neutral & back into drive & nothing. Coasted to the side of the road, tried all gears multiple times & none would catch. Rechecked trans. Fluid - still good. Checked shift rod/lever, it looked fine. Had to be towed the remaining 110 miles. Took transmission pan off replaced filters & fluid. No sludge or metal shavings in pan. The old flat filter had a crack in one side. Thought problem would be fixed then, but no. From the time I was on the side of the road I had noticed a low whirring sound it had never made before, specifically when I was starting it, moving gear shift or accelerating. I finally figured out the whirring noise was coming from inside the housing where the flexplate is. Also when I turn vehicle off you can distinctly hear the whirring winding down. I haven't gotten inside that housing to look yet. Seems a little intimidating of a job for a novice like myself. But I will do if necessary. From what I understand though, if there was a problem with the flexplate itself, or it's connection to the torque converter or drive shaft, there would be a whole lot more noise, (& big problems), going on. Do you think it's more likely to be the pump or the solenoid pack or maybe something like the speed sensor, (which I think will be my next attempt unless someone gives me a reason not to), or do you think it's something else entirely? Oh, also I did try doing a hard reset on the whole electrical system, to no avail.