... are we really meant to be able to know what you have, and what you have done from that one line? how about you give us a little more info to go on? what is your ide of choice (monodevelop or visual studio), how have you created the script? etc.
If you're using VS, sometimes the .NET version throws a popup box and you have to select it on the taskbar to confirm the choice.
Is it giving you an error? If you used an code you found online you may have the script title wrong.. Like this.. public class MainMenu : MonoBehaviour If your script in your inspector is called Menu.. Yet your script says MainMenu then it wont work.
With such little info provided: I'm not sure if he talking about the code editor or launching the game and getting stuck at the loading splash.. So I totally get @shawnrevels questions - the OP is too ambiguous to decide.
Maybe he can download Xamarian studio from the official site then use the plugins from here: http://forum.unity3d.com/threads/unity-add-ins-for-monodevelop-xamarin-studio-5-9.329880/
Can you open Mono by itself ? - not using unity. Probably Mono hanging not related to Unity. Go to the Unity folder in Program Files/Unity, there is a folder - MonoDevelop. Run the mono shortcut in that folder. Does it run ok?
Ok, well if it runs fine leave it running then start unity. When you double click the file it should use the running instance of Mono ... but that is just a work around...
Happens to me occasionally. Mono Develop throws an error which appears behind the splash screen and the splash screen won't go away until I close the error. Sometimes it is a big dialog and you can see it, other times it is small and you can't see anything but the splash screen. Very annoying. Here's a few techniques I've used to get around the issue: Close MonoDevelop and try again. If the error dialog is large enough, you can grab its title bar and move it somewhere the splash screen is not in the way If the error dialog is too small, click the MonoDevelop splash screen to select the error dialog, then hold alt and press the space bar. This brings up the Windows context menu. Press the C button to choose close.* Alternatively, the Esc key sometimes works to close the error. Hopefully there are not too many error dialogs since you will need to do this for each. *For Windows PCs
Mono chokes all the time like this. Steps to cure: 1. Go into task manager, kill any extant MonoDevelop.exe tasks 2. Go into your project root (where Assets and ProjectSettings are) and delete these files: Assembly-CSharp-* (all those files) and pretty much any other file you don't know is yours in that directory (shouldn't really keep anything in there honestly; none of those should be source controlled; Unity rebuilds them). 3. from Unity, use Assets--> Sync MonoDevelop. Unity will regenerate all the above files and launch Mono Edit: to clarify delete-able files in this directory, you can delete files that match these wildcards: *.userprefs *.sln *.csproj *.unityproj ... because Unity rebuilds them when you open a C# file or do Sync Monodevelop.