Samsung TV hangs at "Selecting Network"

Hi and good morning.

I've had a Samsung UE43J5600 TV for a couple of months now and it has been working fine. I have an external USB HDD plugged in which has also been working fine. The TV refused to pick up some new media that I copied to the drive so after checking on the Internet, I did a reset by selecting the option on the Support menu.

When I now switch the TV on, it goes into the initial setup mode but immediately freezes at the Detecting Network screen, i.e. the progress indicator stops animating. I've left the TV at that stage for around 30 minutes hoping it would time out but it didn't.

I've also tried turning the TV on in to the technician menu by pressing Info, Menu, Mute, Power on the remote control - this switches the TV on and gives the Samsung logo but it then stays at a plain black screen.

I've tried leaving the TV unplugged overnight and also removing the USB HDD but that makes no difference. Does anyone have any suggestions at all please?

Thank you,
David.