Open the system up and see if one or both of the antenna connectors to the WiFi card has come loose. Someone else had a problem like this recently. You can find the manual for your system on support.dell.com, which will include the necessary instructions (with diagrams) for how to open your system up. If you want to reseat the antenna connectors, I would remove the WiFi card from its slot so you can get your index finger along the underside of the card directly under the antenna connector. That way you can pinch the antenna onto the connector between your thumb and index finger, then just reinsert the WiFi card into its motherboard slot.