Read the error. Then pick the matching rabbit hole.

Reinstalling the launcher first is how you lose an hour and keep the same Xbox token. Filter the list, open the guide that matches the words on screen, and only wipe folders after worlds are copied out.

Failed to download fileBar sticks, then a red error on a jar, json, or sound. Unable to locate the Java runtimeBundled runtime missing or custom path is dead. Sign-in never finishesStore / Xbox / Gaming Services disagree. Play, then the window diesGame process, GPU, or a bad JVM argument. It will not installStore codes or leftover unified launcher. Linux-specific launch failuresPackage, drivers, then Java profile.

Official next stop

If none of these match, use help.minecraft.net and the Mojang bug tracker (launcher issues use the MCL project). This site is a map, not Mojang support.