How old is your TV set? The apps on smart TVs only work as long as the manufacturer continues to pay the licence fees for those apps. Sony is well-known for discontinuing support (through paying those licence fees) for apps on TVs that are as little as two or three years old. For example, earlier this year they withdrew support for Youtube on their TV sets which were manufactured prior to 2013:
https://support.google.com/youtube/answer/6098135?p=yt_devicesupport&hl=en-GB&rd=1
Perhaps you've suffered a similar withdrawal of service?
Many writers in the technical press warn against buying smart TVs (where you can lose much of the 'smart' functionality within a very short period of time). They advise that it's better to buy third-party 'smart' devices, where the manufacturers are obliged to keep the functionality going in order to maintain their reputation for reliability. For example, a NOW TV box cost £14.99 and adds Youtube (plus catch-up services, such as BBC iPlayer and internet radio services) to
anyTV:
http://www.nowtv.com/box