Why exactly did this happen on a disk that was working perfectly fine the day before?
For the exact same reason your car breaks down on the road when it was working perfectly the day before. In other words C'est la vie.
Now, as to your situation, you've presumed that the drive went bad without ruling out the other factor - the enclosure itself. The USB port would be the third factor, but you automatically ruled that out by going to a different computer.
There's a very similar question to yours already posted:
External Hard Drive no longer working. That one involves an enclosure and an SSD, not an HDD like yours, but the principles are the same.