Always helpful if you tell us what model of Sony TV you have and what software it is running.
Have you tried restarting the TV, long press on the power button on the remote, then select restart.
Or have you tried a full power cycle. Switch off, unplug wait a couple of moments and then switch on,
if these don't resolve the issue try this next.
Go to Settings> Apps> See All Apps select the Channel 4 app.
If Open is available click on that and see if it opens the app.
If not click on Force Stop.
Now click on Open.
Have you checked to see if there is any new firmware for your TV.
If there is install it the process of installing may just refresh the TV and make the app work again.
