The Minecraft launcher said there was an error and to report it here. I opened the launcher, the play button was disabled, and in the development console it said this:

Fatal error launching game. Report this to http://mojang.atlassian.net please!
java.lang.NullPointerException
at net.minecraft.launcher.GameLauncher.getMinecraftArguments(GameLauncher.java:302)
at net.minecraft.launcher.GameLauncher.launchGame(GameLauncher.java:237)
at net.minecraft.launcher.GameLauncher.onDownloadJobFinished(GameLauncher.java:499)
at net.minecraft.launcher.updater.download.DownloadJob.startDownloading(DownloadJob.java:75)
at net.minecraft.launcher.Launcher$1.run(Launcher.java:157)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:439)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
at java.lang.Thread.run(Thread.java:680)