The launcher fails to load Minecraft and then I can't click the 'Play' button again.
This is the code in the console that fails:

[12:59:36 INFO]: Launching game
[12:59:36 INFO]: Looking for old natives & assets to clean up...
[12:59:36 INFO]: Unpacking natives to C:\Users\Robert Kempton\AppData\Roaming\.minecraft\versions\1.7.5\1.7.5-natives-436475173261098
[12:59:36 ERROR]: Couldn't unpack natives!
java.util.zip.ZipException: error in opening zip file
at java.util.zip.ZipFile.open(Native Method) ~[?:1.7.0_51]
at java.util.zip.ZipFile.<init>(Unknown Source) ~[?:1.7.0_51]
at java.util.zip.ZipFile.<init>(Unknown Source) ~[?:1.7.0_51]
at java.util.zip.ZipFile.<init>(Unknown Source) ~[?:1.7.0_51]
at net.minecraft.launcher.GameLauncher.unpackNatives(GameLauncher.java:479) ~[launcher.jar:?]
at net.minecraft.launcher.GameLauncher.launchGame(GameLauncher.java:217) [launcher.jar:?]
at net.minecraft.launcher.GameLauncher.onDownloadJobFinished(GameLauncher.java:629) [launcher.jar:?]
at net.minecraft.launcher.updater.download.DownloadJob.popAndDownload(DownloadJob.java:120) [launcher.jar:?]
at net.minecraft.launcher.updater.download.DownloadJob.access$000(DownloadJob.java:12) [launcher.jar:?]
at net.minecraft.launcher.updater.download.DownloadJob$1.run(DownloadJob.java:89) [launcher.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.7.0_51]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.7.0_51]
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [?:1.7.0_51]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [?:1.7.0_51]
at java.lang.Thread.run(Unknown Source) [?:1.7.0_51]