Game Won't Start

Play greys out, the launch hangs, or the game window never appears.

Launching
  1. Read the bottom-left of the launcher. It says whether Play will work and which step it is on. If it says it is downloading, let it finish; the first launch of a version takes a minute or two.
  2. Signed in? Play needs an account that owns Minecraft: Java Edition. Check the account menu in the top right.
  3. Is a game already running? Without Run more than one game (Sour Gold), only one game runs at a time. Check the taskbar and the system tray.
  4. Antivirus. Some antivirus tools quarantine game or Java files. Check its quarantine list, restore anything from Sour Client, and add an exception for %USERPROFILE%\SourClientData.
  5. Disk space. Keep a couple of GB free on the drive with your data folder.
  6. Repair the files. Settings, turn on Verify files on launch and Repair corrupted files on launch, then Play again.
  7. Java path. Settings, Java runtime: if Java path is anything but java, press Reset.
  8. Your own mods. A wrong-version mod can stop the game before its window appears. Switch your added mods off and try again (crash checklist).
Turn on Show game log console in Settings and launch again: the log window shows exactly where it stops. Send that in a ticket if none of the above helps.