I am using Visual Studio 2013.
For some reason, the Start Debugging button is missing on my toolbar. Also, when I am debugging, the Stop debugging/Pause buttons are
I've found that sometimes these buttons just disappear on me as well.
Here is how to fix the issue. On your main toolbar, select the TOOLS dropdown and select "Customize".
You should see a menu similar to this:

Ensure that the "Standard" and "Debug" items are both checked and click close.
You should now properly see your Start / Pause / Stop buttons.
Note: If this does not solve the issue, you will be forced to reset all of your settings to default. You can do so by going through the TOOLS dropdown, clicking "Import and Export Settings", and selecting the "Reset all settings" option.