how to fix default audio in my pc

Open Device Manager, check if you've audio driver installed.

Try updating audio drivers to latest version.

Or if you've already updated them, try to roll back previous version to see if that works.

Run Playing Audio troubleshooter from Settings app > System > Troubleshoot > Other troubleshooters. Let us know what it reports back.

Your laptop may be experiencing a fatal malfunction. If a clean reinstall of Windows does not restore audio, have it repaired by the manufacturer or purchase an alternative USB audio device.