Hello, thank you for reaching out to Microsoft Q&A.
Could you clarify which input device isn't working? Is it your microphone, or another input device such as your keyboard or mouse? This will help me share the appropriate troubleshooting steps.
If the problem is with the Microphone (Audio input device)
Go to Start > Settings > System > Sounds > Choose your input device.
May I please know the name of default input device selected in the system sound settings?