This always happens to me. Once, when the same thing happened, I just pressed “render” again and it worked. However, that’s if you’re lucky. You should either try rendering right after your computer turns on, so that you have a maximum amount of virtual memory (which could also be another factor) or you could try rendering pieces of your audio or video at a time. One question – are you trying to render multiple audio/video files at once? Because if you are, then Sony Vegas usually freezes up at such strenuous work.