5 possible reasons why.
1. Recent windows upgrades have setup conflicts with your drivers for the USB port, rendering it not working.
2. The port has up and quit, due to age or busted connection inside the laptop itself.
3. The adapter has up and quit, due to age or busted connection inside the adapter.
4. Due to recent Windows upgrades, your sound/audio settings are reset and the USB currently isn't set for audio defaults.
5. The headphones themselves are bunk, so the USB port might not recognize there's actually a connection being made.
Without knowing exactly what's been tried, what's been tested, what still works fine and what doesn't, there's no definitive answer possible.
Usually the microphone is listed under recording...