If bottom right of the screen you have a Red Cross like this
Then check the cable, and every physical connection. 95% of the time this indicate that the cable is not plugged in.Or you can try disabling and then reactivating your network Ethernet card
If bottom right of the screen you have a Yellow Triangle like this
Then Check This :
Right Click on Ethernet and go Properties
Select IPv4 and go Properties again
Check that Obtain Ip Adress Automatically is checked, as well as Obtain DNS server address automatically
