I don't know why minecraft wont work after a 1.12 update but a text that comes up when it doesn't work says:
Set run directory to C:\Program Files (x86)\Minecraft
Native Launcher Version: 1321
Operating System: Microsoft Windows XP
Application Hash: d026a01a83aeaec4111e8c7430ad89b21f3a30f3
Application Data directory: C:\Users\Camren\AppData\Roaming/.minecraft
Executable Path: MinecraftLauncher.exe
App Directory dir: C:\Program Files (x86)\Minecraft
Start dir: C:/Users/Camren/AppData/Roaming/.minecraft
Launcher dir: C:/Program Files (x86)/Minecraft/game
TmpDir dir: C:/Program Files (x86)/Minecraft/tmp
(Optional) Java Launcher hash:
Java Version: 1.8.0_25
x64: 1
LauncherConfiguration from: http://launchermeta.mojang.com/mc/launcher.json
Downloaded config
Native launcher is current
Native launcher is current
Using new launcher as a launcher.dll is on disk.
Launcher library is up to date with hash ec064f05257ba720c432bce019329ec56417a61f
Check to see if we can run the launcher
Native launcher is current
Starting launcher.dll with C:\Program Files (x86)\Minecraft\runtime\jre-x64\1.8.0_25\bin\javaw.exe
Found SetDllDirectoryW in kernel32.dll!
Added C:\Program Files (x86)\Minecraft\game to DLL search path
Running launcher!
2:48 PM 6/28/2017