I have noticed this problem recently on quite a few of the Dell Optiplex 3050 models that I manage, and it seems to be caused by the latest Windows 10 creators update (version 1803, Spring 2018). The problem actually seems to occur just BEFORE the update is actually installed, presumably due to a network driver update that is installed before the major update.
The symptoms are that your network port shows a link and is connected, but you will have the yellow error indicator on your network icon in the system tray. You will not receive an IP address (if you are using DHCP), and if you are using a static IP address, you will still see that you have the correct IP address, but you just won’t be able to connect to anything.
Luckily the solution to this problem is very simple. Go into device manager Device Manager, and expand the “Network Adapters” section. Right-click on the “Realtek PCIe GBE Family Controller” and select “Uninstall”.
In the windows that pops up, also select “Delete the driver software for this device.” and press OK.
Now, restart your computer and it will automatically re-install the network drivers, and your network connection should be working normally again. However, if you had a static IP address configured before, you will need to go back in and reconfigure that information after doing this.