Is the issue for you or your student to resolve?
Firstly, are your headphones plugged in?
If you can't hear any of your students, it's likely the issue is on your end.
If you can hear most students, then it's more likely an issue for your student to solve.
If your headphones are plugged in and the issue is most likely with your computer, then the most likely cause is that your computer is confused by whether it should be using your headphones or computer speakers.
Try a couple of easy solutions: