This seems to be an annoying problem with Dell laptops. It happens enough that it appears to have been a planned "feature." If you turn on your Dell laptop, and there is no power cord plugged in (you are using battery power) and no network cable plugged in, the hard-wired network device does not appear. We have three (different model) Dell laptops and we have seen this on each of them.
Usually, you can get the network adapter driver to reappear simply by shutting down the laptop, plugging in the power cable and a live network cable, and then powering the system back up. Only once have I had to actually reinstall the driver.
What's annoying is that the network adapter driver randomly disappears if the network cable isn't plugged in during power up, or if the power cable isn't plugged in during power up. It isn't really gone, since rebooting with both power cable and a live network cable plugged in usually reactivates the driver.
My previous laptop was a Gateway, and this was never an issue. Dell needs to fix this problem so that customers can successfully plug in a network cable when running on battery power after the system has already booted up.