What happened? Did it break? Did I accidentally uninstall it?
Perhaps you've asked yourself this before. You open up the render settings window and try to switch the renderer to Mental Ray only to find it missing from the drop down box. You see Maya hardware and software. Maybe even the vector renderer. But Mental Ray is nowhere to be found.
Is it a bug? Yes and no.
Maya gives you the option of turning different features on and off in the plug-in manager. (
Window>Settings/Preferences>Plugin Manager). Mental Ray is one of the features included in the list.

You can turn it on and off manually, but for some reason, it occassionally turns itself off - usually after a crash. (This is a bug that has supposedly been fixed, but I've seen it in every version of Maya I've ever used).
While we're on the topic of missing plug-ins, there's quite a few other features that you can enable or disable in the plug-in menu (such as a variety of shaders and import/export for obj, fbx, and anim), so if something's missing, it's generally a good place to look.
On the other hand, if there is a plug-in that you don't really use or need, keep it disabled in the meantime and save yourself some RAM. For more on that, read this old post on
Paint Effects and Dynamics.