Please do try these troubleshooting steps that may help in solving the issue.
- Start by uninstalling/reinstalling the wifi adapter.
- Go to Device Manager and uninstall the wifi adapter.
- Next is to download and install the latest driver from Dell's Support site.
- Here's the link where you can download it from. (use the Windows 7 drivers)
- Once installed reboot your for all the changes to take effect.
- Do test your laptop again if it will connect to your wireless network.
- If these doesn't work do toggle the shortcut key to enable/disable the wifi adapter until the wifi ligh turns on.