What is the current OS installed on the system?
Is the wireless card detected under network adapters in device manager? Does the ethernet work fine?
Restart and press f2 on startup to enter BIOS - check if the wireless is enabled in BIOS.
If the system was working fine earlier, then perform a system restore to restore the machine to an earlier point where it was working fine - http://bit.ly/1JEw2ov
Restart and check in safe mode with networking.
Try connecting to a different network and check if it works.
Create a hotspot on your Android smartphone and check if the system detects and connects to the phone.
Update the BIOS - http://dell.to/1n3X2po - enter the service tag and download the BIOS, update and restart the machine and check.
Restart the system and press f12 key on startup - choose diagnostics. Let it run the diags and if you encounter any errors - make a note of the complete error and report the same to us. These are onboard diagnostics and they might possible pick some error if the hardware is not functional.
Also, please click my DELL-username and write me a private message with the service tag and your Name for case records.
Let us know if you have any other queries.