Just went through a hair-pulling hour trying to figure out why a sequence that had rendered in the past was suddenly not rendering anymore. The footage is HD DSLR footage at 1080p, with a separate audio track. Simple cuts and effects, and an adjustment layer for color-correction.
I had just successfully compiled another clip in the project and the only difference I could see was that at some point I had added a cross disolve from a Title clip to the Adjustment layer that was in the same (Video 2) track. Not sure why I did this, but it must have gotten dropped down from track 3.
I removed that cross disolve by moving the Adjustment layer to a new Video track, and fixing the Title's fade out. Sure enough, it works!
It's a stupid error, but if you find yourself in the same situation, hopefully this could help.
Stephen