The game will not start, or the launcher is stuck

1. Close the game and the launcher completely, then open the launcher again. A stalled update usually resumes.
2. Run a repair from the launcher. It checks the installed files against what the update was supposed to deliver and fetches back anything damaged or missing, without a full reinstall.
3. If the launcher itself will not start or cannot update, install it again from the download page over the top. Your installed game files are not thrown away.
4. Check the status page before assuming it is your machine. If a world is down or in maintenance, that page says so and says why.

If you open a ticket about this

Tell us your operating system, whether the launcher or the game is the thing that fails, and what happens exactly: a message, a window that closes, or nothing at all. That is usually the difference between one reply and five.