Look in your Package Explorer and check if you see a Red Exclamation point such as below. 

If so, go here:

If Minecraft is not running then you can check on the error in the console. The console is the text box that appears when you hit the run button and fills with gibberish. It's actually telling you what it is doing behind the scenes, and most importantly, what happened when it crashed. Take a look at the console and scroll up a bit. 

If you see this:

Go to this link.

If you see this:

Go to this link:

Is your fix not shown here? Please reply to this article and copy and paste everything from your console into a message to support. We will be happy to help.