There are three possible problems if you're missing a network adapter:
The network adapter isn't installed properly – This usually only happens if you've just installed a new adapter or other new hardware and have knocked the network adapter loose in the process. There's a driver issue – This can happen when you first install a network adapter in Windows, after Windows Updates, and when upgrading to a new version of Windows. If Windows isn't installing the right driver automatically, check the network adapter manufacturer for an updated driver. The network adapter has gone bad – No PC hardware is immune from wearing out. Newer adapters may have a defect, your PC may have overheating issues, or it's simply worn out (older adapters only).
If your network adapter isn't missing, but there's an error, the tool you choose to use may show you more info about the error. You can also check Event Viewer (Start -> type "Event Viewer" -> Select "Event Viewer"). Read this tutorial to find the fix for network cable unplugged errors.