I have changed broadband provider from sky to another one. I have setup the new WiFi code on my tv,

[Clare40]

I have changed broadband provider from sky to another one. I have setup the new WiFi code on my tv, but when I open apps, eg. You tube, the picture disappears and I just have sound.  This also happened with bbc iplayer and netflix, although, they seem to be OK now. 

While I had no Internet connected in the week, the screen kept going blank and I would only have sound. Turning the TV off and then on again worked. I have tried this today since connecting to Internet but the screen is still going blank. Please advice on what to do next. Many thanks.