Sony Vegas Confusion!!!

Hi.. i have been using sony vegas movie studio for about a year. i don't use it that often and I am pretty much a self taught computer girl. The "instructional" books that come with the software are vague, to say the least, and i have gotten stumped....I have rendered 3 different video "movies" for my employer in the past. I went to open one of them up the other day and the files are no longer ready for burning. I burnt 300 movie cd's in august so i know it all worked then...i made sure it wasn't a fluke by checking the other two videos i have completed and all of them are the same....WHY IS IT NOT SAVED AS A COMPLETED MOVIE FILE? is there any way to save my work so that It is forever ready to just burn the movie or do i have to go through the agonizing render process each time?