I'm having the same problem everyone else is having. The recommended solutions are not working. Samsung caused the problem. Samsung needs to fix the problem. Model UN55JU6400. All downloaded apps deleted and can't download new apps since update.
Right, let's do something else now. Power your TV off and wait 30 seconds, after this power the TV back on and wait 5 minutes. During this time do not press the Smart Hub button. Once you have waited 5 minutes, power the TV off at the mains and wait another 30 seconds. Should this not help, you can perform a Smart Hub Reset on your TV.
You can find how to do this on this handy guide here