On occasion one might press a shortcut on the keyboard, which might change some setting. If this, occurs the easiest fix is often to reset the program to factory settings.
There is only one way to reset user preferences in Maya, and that is to remove the userPrefs file located at:
(Replace version with the installed version of Maya i.e. 2010 or 2011)
Maya will create a new one on startup with the default settings.