In Windows 7:
1. Network adapter get disabled after resuming from sleep mode (wake up from sleep).
2. Network adapter not visible in device manager until the LAN cable is plugged.
LAN adapter will be missing in Device manager and Network and Sharing Center. When we plug the LAN cable then only it appears and gets connect.
1. Network power management is applied to network adapter.
2. Outdated Ethernet card driver.
Plug the LAN cable
Go to device manager and expand the Network adapter
Right click on your Ethernet card and click properties.
Click on Power Management and remove the tick mark from "Allow computer to turn off this device to save power"
Above Solution Is Applicable for All Laptops with Window 7 OS