Why can i only open minecraft once?

Ok do this - Hold Ctrl, Alt, and Delete all at once, and you should see a few options, select task manager, this should bring up a box with multiple programs that are currently running listed in it. Do this every time you exit the game ( making sure to quit the game through the menus no just clicking the x on the top right if in window mode. ) If minecraft is still running it should be within the list on task manager. Click it and end the task. This will insure the game is completely shut off so next time it can startup properly. If this does not help then hopefully you find the correct solution or contact their help line or such to fix it.

Try the Magic Launcher