You probably need to do a Force Quit. Swipe up from the bottom of the screen so that all open apps can be seen as rectangles. Touch the one you want to quit and slide it off the screen. Tap anywhere on screen to get back to Home. Then try again to see if the app will open.
Applications (apps) need to buffer, i.e. they have to preload data in a reserved area of memory (know as RAM, Random Access Memory) before they will run. More often than not you won’t notice this but ITVX is known to have buffering issues and users are aware of this. It’ll probably be because your device doesn’t have a lot of RAM and what there is is being used by other apps.
ITVX is one of the slower apps to load on my smart TV but once loaded it doesn’t buffer. It loads almost instantaneously on my MacBook which has 16GBb RAM. To compare, a sixth generation iPad launched in 2018 only has 2Gb of RAM.
If your device doesn’t have a lot of RAM and you are also running a lot of other applications in the background, it will struggle. If you have a browser running, Firefox, Safari etc anything open via that will slow things down, especially by those sites which have dynamic moving content e.g. Gransnet and its video adverts. Tabloid newpapers have a lot of video pop ups too. You can use an adblocker but then the adblocker has to be constantly running in the background to stop the ads.
Try closing everything else that’s running see if it makes a difference to how ITVX loads. If you have Activity Monitor, look at that to see what is using up memory. It’s an integral app on MacBooks but may have to be downloaded as a separate app for iPads.