My External Hard Drive's format changed to FAT16 without any reason

It seems somehow the partition table (and possibly more) was corrupted on your hard drive, as you indicate that the drive is not readable anymore in macOS or from Windows.

The next step is to stop touching the drive at all before you have made a bit-for-bit image copy of everything on it to secondary storage. After that you can work on copies of that image in order to try to rescue your data.

It might be that you accidentally overwrote the partition table unknowingly. Then data recovery is probably fairly easy. It might be that your drive is starting to fail and various errors pop up sporadically - this is usually much, much harder to recover data from.

In order to create a bit-for-bit copy of the drive, you'll need to find the disk number from Disk Utility. In your screenshot it is currently listed as "disk2", please ensure that you double check this before proceeding.

When you have the disk number, open Terminal and run the following command:

dd if=/dev/rdisk2 of=my_image_file.img bs=512 conv=noerror,sync

The command probably takes a long while to complete - creating a new file named "my_image_file.img" containing a complete copy of your drive. Make sure that you have space enough to store the file. If you haven't got room on your internal drive, you can plug in another external drive and store the file there.

The "noerror" and "sync" options to dd makes sure that if there are drive read errors (i.e. the drive is actually physically failing) then dd will try to skip the errors, replacing those with bytes containing zeros. This won't get you the data back ofcourse, but it makes a lot easier to recover the rest of the data.

When you have the image copy, you can use any data extraction / data recovery software to work on the image to try to find your partitions, file systems, or even single files that you want to extract.