Minecraft Won't Lauch / Open

I have been trying to play minecraft but it won't work. This is what is happening:
1: I start up the minecraft launcher.
2: I log in.
3: I press play.
4: As soon as the loading is done, the launcher disappears (normal).
The icon for the game shows up for a millisecond & disappears with the launcher then reappearing.
I have NO mods, completely clean .minecraft and java and everything is installed. It worked before about about 2 weeks ago and the last few times I have tried to open it, it won't open the game.