There are tons of reasons, why MR could aborted your rendering. Wrong texture files like lzw compressed tiff's, geometry, that can't be tesselated, and and and... You have to search for some clues first, befor someone can really help you. Open your Script Editor (icon at the right bottom) and look, if there are any informations, what happend befor the exception. Also you could take a look in the Render Log. On Win it's under "C:\Documents and Settings\USERNAME\My Documents\maya\mayaRenderLog.txt"
Try "Render/Render diagnostics" and look in ScriptEditor for any warnings or errors.
Last but not least u can use "File/Optimize Scene size" to get rid of all the crap like invalid Nurbs, unused stuff and empty transforms.
But do this with caution ! The warning, that this is not an undoable action (at least in m6.5) isn't for nothing. And really look at the deleted overview (also in ScriptEditor - btw this is a real friend! ), if there wasn't anything, that you didn't want to be deleted. (Perhaps a shader, prepared but not yet assigned to geometry, etc)