Difficult from a distance

1. Check the Windows soundcard settings to make sure it is outputting to the correct terminals
2. Most soundcards have an eq setting somewhere in their software so make sure a program hasn’t altered this.
3. The soundcard has become defective
4. The cable has become defective
5. The Tweeters have become defective
6. The amp feeding the tweeters has become defective

It’s a matter of swapping things out to locate the problem, however my guess is either settings have changed or the Tweeter amp has failed.

Hope this helps

Bill