Exit code -1 is Minecraft's generic launch failure code, which means the game closed during startup without a specific error to report. Because the code itself names no cause, the fix starts with the crash report and the game log, which usually name the real problem, such as a mod conflict, the wrong Java version, too little RAM, or corrupt game files.
The fix, in order
Read the crash report. Open the .minecraft folder, then crash-reports, and open the newest file. The Description line and the first Caused by line name the problem, often a mod ID or a memory error.
Check the launcher log as well. The latest.log file in the .minecraft logs folder records the last lines before the crash, which can point at the failing mod or file.
Test with a clean, unmodded game. Move the mods folder aside, remove shader packs and resource packs, then launch. If the clean game works, add mods back a few at a time.
Check the RAM allocation. In the launcher, open Installations, edit the profile, open More Options, and confirm the -Xmx value is reasonable, such as 4G for a modded game, and not larger than the free memory on the PC.
Reinstall the launcher's Java runtime. The official launcher bundles its own Java, so reinstalling the launcher refreshes it. Do not install a separate Java unless the log says the Java version is wrong.
Verify the game files. In the launcher, check the installation, or reinstall the game to replace corrupt files.
Update the graphics driver from NVIDIA, AMD, or Intel, since a broken driver can also stop the game during startup.
If it still happens
If the crash report names a mod, update or remove that mod, and check that every mod matches the game version and the mod loader. If there is no crash report at all, the game closed before it could write one, so check the launcher log and test the clean game first.
Common questions
Why does exit code -1 have no message? It is a catch-all code for a launch that failed without a specific error. The detail lives in the crash report and the log, which is why reading those files is the first step.
Can exit code -1 mean the PC ran out of memory? Yes, it can. On some systems the operating system kills the game when memory runs out, which shows up as exit code -1. If the log mentions memory, raise the RAM allocation and close other programs.