There may be some possibilities why you are getting this problem.
1. Your network cable may have been actually plugged out of your LAN port of your PC. Check for the connection and ensure it is not loose. If it is not the case, go for step 2.
2. The connection of the LAN cable with your router may have been disturbed. Check the connection. If it is not the case, go for 3rd probability.
3. Your router may have been turned off. Check for the plug of the router and make sure that the electricity connection is OK. If it is not the case, go for fourth one.
4. May be there is some problem with your LAN driver. Try reinstalling the LAN driver.
