Close Java in the task manager, reboot, and try again. This sometimes happens to me if I have Java running in the background (i.e. if I accidentally click the MCPC+ jar instead of start_server.bat and it starts my server without a command line).
Just press Ctrl+alt+delete, click "Task manager", go to the Process tab and find java.exe or javaw.exe. Right click it and end it. If there are multiple end all of them.
In your .minecraft folder, find a folder called "crash-reports". If there are files in it, post the text most recent one in spoilers for us to see. If not try redownloading the Minecraft launcher.
EDIT: There seems to be a way you can get a crash log even if Minecraft(the game process, not the launcher) never starts.
1. Browse to the folder that contains your minecraft.exe launcher
2. With no files selected (left click on background to deselect), Shift+Right Click anywhere in background of the window, and choose "Open command window here"
3. Copy the following code:
"%ProgramFiles%/Java/jre7/bin/java" -Xms1G -Xmx1G -cp Minecraft.exe net.minecraft.LauncherFrame"
4. Right click in the command window, select Paste and then press enter
5. Login, start Single or Multiplayer and reproduce the crash
6. Once you have obtained the crash report, right click on it, select all and press Enter to copy, then paste it to the proper mod thread for assistance
Error: Could not find or load main class net.minecraft.LauncherFrame (it's in the old launcher, but not the new launcher)
The game worked a couple hours ago, but now it just closes.
What do I do?
No java.exe or javaw.exe was in there.
Error: Could not find or load main class net.minecraft.LauncherFrame (it's in the old launcher, but not the new launcher)
LiteLoader was messing with it.