The main question at this point is whether the hardware is damaged. Usually the main thing that destroys chips is driving the power supply or other pins too high, or with a negative voltage.
But there are many other ways to stress the hardware, which often do not result in immediate failure, but can damage the chip. Shorting the pins to GND or 5V and attempting drive the pin to the opposite logic level can cause excessive current flow. From
your other thread, looks pretty likely that happened at some point. Whether that killed your chip, I don't know. It certainly isn't good and can impact the long-term reliability. I can tell you I've personally made that mistake many times while experimenting and rarely does the hardware die immediately, even though the pin's current goes far beyond the rated specs.
Anyway, the very first thing to do is try to establish if the hardware is destroyed. Assuming you have good 5V power, and if it's not showing any signs of running whatever you previously programmed, and pressing the pushbutton doesn't give any response in the Teensy Loader window, I'm afraid the signs aren't good.
One final test to try is fresh reboot of your PC without the harwdware connected and then hold the pushbutton down while you plug in the USB cable. Get Teensy Loader on your screen before you plug in the cable, and make sure Auto mode is turned off (so you can easily see if the hardware gets detected). It's a long-shot, and usually that last resort test made a difference in the times before Microsoft finally published good USB drivers in Windows 10. But it is one last thing to try.
And I know you said in msg #4 that you're sure your USB cable is good. But I'm going to bring it up again, because sometimes after a lot of troubleshooting effort
the problem does in the end turn out to be a low quality USB cable. (and unlike flash memory which is rated for 100,000 cycles, good quality USB cables are rated for 5000 insertion cycles and cheap ones probably survive much less)
But in the end, if the hardware really is dead, there's simply not much point in spending a lot more time troubleshooting. There is no practical way to fix a chip that's been internally damaged.
Sadly, Teensy++ 2.0 was discontinued earlier this year, partly because it's very old, partly because sales on those old models slowed to a trickle over the last few years, but mostly because the chips aren't really available anymore. Tinkersphere might still have some but
at an outrageous price. Before you go that path, maybe start a new thread and ask if anyone on this forum still has an extra they might be willing to sell.
Wish I had better news or a magic solution, but sadly this is the best I can do.