Built-in MonoDevelop won't launch

I have the built-in MonoDevelop selected from preferences, but whenever I double-click a script file the system default editor for .js files pops up. I’m using Unity 3.5 and 64-bit Windows 7. What do I do?

This happened to me recently as well. MonoDevelop started giving errors on every tab-change.
Quitting by using the ‘X’-button in the upper corner and restart it did not help.
Every time I reopened it, it would produce just an empty grey background.

The Solution for me was to force quit the program through the Task Manager:

  • On windows, press CTRL+SHIFT+ESC. (or right click the task-bar and click “Start Task Manager”) This opens your Windows Task Manager.
  • Click the “Processes”-tab in the top, and click the “Image name” to sort the list by name.
  • Now look through the list for “MonoDevelop*”, highlight it and click “End Process”.
  • Once it is gone from the list, you can go back to Unity and double click a script. It should now open like normally.

On a side-note, restarting the computer will also be sufficient.