(adsbygoogle = window.adsbygoogle || []).push({}); why does my tnt go away?

If you are on a server it is most likely because they have TNT disabled, if it is in your singleplayer then I am not sure sorry. Maybe a mod you have installed is interfering with the tnt?

When exactly does it go away, when you try to ignite it?
If so it's cause the new update made it so that you have to use a flame or redstone

You need to use a redstone circuit, or you can put a pressure plate on top of it or you can right click it with flint and steel, heres a link to a vid with a tutorial,