I'm having the same issues here. Bought our Smart TV in 2012, in 2013 we used Netflix with no problem, WiFi was working fine.
6 months later, it stopped detecting Wi-Fi signal. We tried a WiFi signal expander, (didn't detect it still, even after connecting the expander to the TV through Ethernet Port), yes the signal on the house was improved, but the TV still didn't detect it. It was kind of a cheap expander (around $30), so I don't know if a better one would fix the problem (I saw someone suggest a NetGear which costs around $60-$80). I'd be happy to try but I'm not sure this would do it, because I connected our Blu-Ray to the expander and it did work, so it should work on the TV too right? (Although the TV is farther from the router). I'm just hesitant about spending on another expander just to find out it wouldn't solve the problem.
After hours on phone with Samsung Tech Support, they told me that my Wireless Card was probably broken, due to power variations, and they suggested taking the TV to a repair center. Since my TV is 3 years old, I'm assuming this is not gonna be covered by warranty, so at least I'm calculating $200 to fix it. Waaay too much honestly. Any of you guys think connecting a WiFi wireless adapter to the TV (through USB) would work? I'm not sure it would override the internal LAN adapter TV has. 🙁 (P.S. We did all the updates etc, etc,)