I've had a two circuit track light for years. Each circuit has 3-way switches. And one of the three ways on each circuit is a dimmer.

For years and years, one set of switches turns on half the lights, and the other set turn on the other half

All of a sudden, either switch is activating the same two lights. And the others remain dark no matter which switch I am using. If I have both switches turned on, then all lights work. But either one of them will kill most of the lights and only leave the same two lights running. So somehow there's a goofy cross-connect.

I am also hearing a buzzing sound in part of the track.

Any suggestions on what could be the cause and the solution?

My suggestion is to call a local Licensed Electrician to troubleshoot. The 'buzzing' requires attention ASAP.

One thing you can do as a homeowner is remove all of the lights and start putting them back one at a time to see if that is your problem. One may be cocked in the track and shorting the 2 segments together or something.

What Greg said. We had the exact same issue at a house and re-positioning the lights just an inch or so from their original positions on the tracks solved it.