Apart from some resource issue you maybe running into or some other reason, I have no idea.
If you HAVE had success in the past, what are you NOW doing differently? What Media are you using? There is much that could throw you off course, but without some "work" on your side by providing some more info - aint much people here can do but move on to another thread. Why don't you say what format the Media you are trying to render is in AND what template are you using? - yeah? Doesn't tell me much?
A] Screen grab of what you are doing
B] What Media mixes and types are you using and render template?
C] Did you create the Veg in Vegas5?
You gotta give us more - yeah? More than - "Every time I try to render this video, I get an error message . . "