If you are using wired Headphones, unplug them from the computer > wait for 15 seconds > Plug the Headphones back in and Restart your computer. If this does not help, move to troubleshooting steps as provided below. If Sound Icon is visible, click on the Sound Icon and try to increase the volume.
If the Sound Icon is missing, you can follow the steps provided in this guide: How to Restore Missing Volume Icon in Windows 10 Taskbar.