I have found the problem. The McAfee antivirus had expired and must have been blocking the connection. After I uninstalled it, it worked fine.    
My one month old Inspiron N5010 has suddenly stopped connecting to the internet either through a wireless connection or a wired one. When I diagnose the error it states that the default gateway is not available. However the router being used is accessible by all other pc's & devices in the house so the problem seems limited to the Dell itself. I have been through the diagnostics and have not found any errors, though there doesn't really seem to be any real tests for network cards. The wireless connection always shows as Connected or limited access, but shows No internet access. Likewise the local area connection seems OK other than showing No Internet access. Can anyone help? I need to know whether to call Dell or not. Is this a hardware issue?   Thanks for your help.      
No Internet access (Inspiron N5010)